Crush on Your Sister's Boyfriend or Brother's Girlfriend?
Entry No. 083 ·
By The IF Team
If you have a crush on your sister’s boyfriend or your brother’s girlfriend, you have not done anything wrong. Feeling something is not the same as doing something, and this happens to people who love their siblings dearly. What sets it apart is that family is permanent. You cannot drift away from someone who comes to Sunday dinner, so your choices carry further: keep the feeling close and your time with them in company, tell them and accept what that could stir up at every gathering, or say it once in a sealed message that waits until you are both single.
The one thing that is never an option is going behind your sibling’s back: cheating hurts people, and so does lying. Everything here keeps faith with your sibling, and so does IF, the dating app we built for feelings like yours: no conversation can open on IF while either of you is marked as in a relationship, yet you can still put the feeling into words.
It usually creeps up on you. He is the one who actually listens to your mum’s stories. She gets your jokes faster than your brother ever did. You were glad your sibling had found someone good, until you caught yourself looking forward to family dinners for a reason you had not expected. You are not broken, and you are not alone.
This is a crush on someone who is taken with a family attached, and the advice holds whoever is dating whom. If you are the one in a relationship and the person on your mind is your partner’s brother or sister, feelings for your partner’s sibling have their own guide.
Why your sibling’s partner, of all people
Birthdays, holidays, getting another bottle of wine from the kitchen at the end of a long dinner: over the years you may spend more easy, unhurried time with your sibling’s partner than with many of your friends, and closeness grows in exactly those conditions. In one set of experiments with pairs of same-sex strangers, the longer they interacted, the more they liked each other. Liking is not romance, but it is the soil a crush grows in.
They also arrive pre-approved. Your sibling chose them and your parents have warmed to them, and being visibly chosen can be attractive in itself: in one experiment, some single participants showed more interest in pursuing someone described as already in a relationship than the same person described as single.
And you grew up laughing at the same things as your sibling, so it would be stranger if your tastes never overlapped. It is worth asking, gently, whether some of the pull is about your sibling too. That does not make the feeling fake. It can make it louder.
Plan for years, not weeks
With most taken crushes, time and distance do the work. Family does not drift. If this relationship lasts, the person you have feelings for may be at every wedding and fortieth for the rest of your life. If something happened between you behind your sibling’s back, it would hurt them, and it would shadow every gathering after it.
That points to a gentler goal. You do not need the feeling gone by Christmas, or to pretend it was never there. You need a way to carry it that lets every family occasion go on as it should. For many people, that means putting it into words somewhere it can wait, and letting it rest.
Should you tell them?
You can tell them. Emotional honesty is brave, and some people would rather know. But inside a family, a confession cannot be taken back. If they do not feel the same, they carry something they must either keep from your sibling or tell them, and it could resurface at gatherings for decades. If they do, nothing could begin honestly while they are with your sibling. You are potentially better off sending an IF: it says exactly how you feel, puts no one on the spot, and if the timing ever lines up, you are there to show them how much you have always liked them.
Keep your time with them in company
Keep to groups
- Sit next to your sibling, not across the table from their partner.
- Top up the drinks, walk the dog, take your nieces and nephews to the park: anything that keeps you among other people.
- Skip the lift home, the errand for two, the “I’ll wait with you for the taxi”.
- Keep your messages in the family group chat, and let any private thread go quiet.
- Mute them on social media for a season. Nobody needs to notice.
Be careful about becoming their confidant
The trickiest role here is go-between: the person they ask what your sister is really upset about, or talk to after a row with your brother. It feels like closeness, because it is, and there is nothing wrong with them for reaching out. But it puts you between two people you love. Point them back to your sibling, kindly: “You should probably say that to her.”
Tell someone
Feelings kept entirely secret tend to grow in the dark. Say it out loud to someone you trust — a close friend, a cousin you have always confided in — or write it down. In a family, news travels fast, so choose with care.
Should you tell your sibling?
That is your call. A feeling is not a secret you owe anyone, nor a confession you are obliged to make. Some siblings would rather know; others would spend every family dinner wondering whether to watch you, and whether to trust a partner who has done nothing wrong. If they ask you outright whether you have feelings for their partner, do not lie: say yes, tell them you have kept your distance, and be honest about anything you have said or sent, an IF included.
If it is your sister’s boyfriend
Sometimes the feeling arrives tangled up with a lifetime of comparison — the sensible one, the clever one, the one who settled down first — and that history can make her boyfriend look like a prize as much as a person. Two moments will test you most.
The first is when she complains about him. Before you agree, check whether you would say the same about a man you felt nothing for. If you truly believe he doesn’t treat her well, tell her, and keep it to what he does and how she seems, never a word that hints at what you might offer instead. The second is when she asks what you really think: whether he is the one, whether she should move in with him. Answer about her happiness, as if you had nothing at stake. If you cannot yet do that honestly, “You know him far better than I do” is kind, true and enough.
If it is your brother’s girlfriend
If your brother’s girlfriend is still new to the family, she may be looking for an ally. If you are the sibling she finds easiest to talk to, you can slide into being her person in the family without either of you meaning anything by it. Step back from that role gently: pull your brother into the conversation and let your parents get to know her directly. If your brother has always seemed to be the one who won, notice whether wanting her is partly wanting to win something back. The same holds for your sister’s girlfriend or your brother’s boyfriend.
If they get engaged
An engagement turns a crush into a countdown, and a sibling’s wedding is the one you cannot quietly skip. You may be asked to be a bridesmaid or best man or to give a speech, and turning it down invites the question of why. Unless you genuinely cannot get through the day, say yes and plan around it: write the speech about your sibling rather than the romance, and leave before the last dance. If you want it said “before it is too late”, an IF says it without asking anyone to decide anything. If the news hits hard, read loving someone who is getting married.
If it is your sister’s husband or your brother’s wife
Being attracted to your sister’s husband or your brother’s wife — your brother-in-law or sister-in-law — can feel like the most shameful version of this, which is why so few people say it out loud. It is not shameful. It is human. A marriage may bring children, a shared home and decades of family occasions, so the principle stays the same while the edges grow clearer. Stay somewhere else rather than a week in their spare room, keep your time with them to the moments the family shares, and stay out of their disagreements entirely. If you want it said, an IF can hold it without touching anything they have built: it stays sealed unless you are both single one day, and then you can show them you felt it all along.
And if you are married or with someone yourself, the feeling is just as honest; what matters is that nothing runs on in secret. Having IF on your phone is not cheating, and an IF can be sent honestly whatever you feel about your own relationship, because nothing opens until you are both single. There is more in having a crush while you are in a relationship.
What if they seem to like you back?
The hug at the front door that lasts a moment longer, the text after a family dinner that is not about the dinner: it can seem to settle the question. It may mean something, and it may not. Adults in exclusive relationships commonly develop crushes on other people, so warmth, even warmth that runs both ways, does not mean anyone has done anything wrong, and is not on its own a sign that your sibling’s relationship is in trouble. For help reading it, see the signs that someone in a relationship likes you.
If it does run both ways, keep it honest by keeping it in the open: no meetings your sibling does not know about, no messages that would need deleting. Whether they stay with your sibling is their decision, not yours to nudge. What you can do is write it down in an IF and let it wait. If the timing ever lines up, you are already there.
If they break up
When it ends, your sibling comes first, and their heartbreak is not your opening. For a long while your job is to be on their side, and never to defend the ex out of some private loyalty. If you have sent an IF, nothing reveals without your say-so: if you are single when they become single, you get 24 hours to reveal it or let it lapse and stay hidden for good, so it never has to land in the middle of their heartbreak. Waiting is legitimate too, and some stories find their way back years later, with a sibling’s honest blessing.
When the feeling will not fade
Most crushes like this soften with time. If you are checking their social media every day, planning visits around seeing them, or dreading gatherings you used to love, it may have grown into something heavier. That does not make you a bad sibling, and you do not have to manage it alone: talking to someone can help. Setting it down in an IF can help too, because once said, it no longer needs rehearsing at every dinner.
Where IF fits
IF is a dating app for everyone, single or in a relationship, built so that you can say how you feel whatever the timing — even when the person you feel it for sits at your family’s table. Two single people use it like any dating app. While either of you is marked as in a relationship, no Like, match or chat can open, so nothing can start with your sibling’s partner on IF. What you can send is an IF: a private, sealed message of up to 250 characters that opens only once you are both single.
For a crush inside your family, that is the release valve. You say it once, honestly, and it asks nothing of anyone today. Your sibling’s relationship carries on untouched, and the feeling no longer has to sit unspoken across the table. If they become single while you are single, you get 24 hours to reveal it. If you do, they see who you are and what you wrote, and choose whether to accept, and accepting opens a chat with your IF as its first message. And if you are in a relationship yourself, you can still send one: it simply waits until you are both single. How an IF works covers every situation.
It is worth knowing how it lands. If they are on IF, they will usually be told an IF is waiting — never who sent it or what it says. If they are not on IF, an IF sent to an email address they gave you triggers one short invitation email, while one sent to a phone number or Instagram handle simply waits in case they join. IF never discloses a sender before a reveal, but in a family, people can guess from context. The choice is yours.
Your first IF is free. Writing one takes a little emotional bravery, and with a family around the table, that is no small thing. Mean every word, let it do the waiting while you get on with your life, and maybe it leads you to where you were meant to be all along.
Questions people ask
Is it wrong to have a crush on my sister’s boyfriend?
No. You did not choose it, and years of family occasions make it easy to grow. Having the feeling, and even admitting it, is honest. What would be wrong is cheating or lying: anything that goes on behind your sister’s back, or quietly trying to come between them.
Should I tell my brother’s girlfriend how I feel?
You can, and it is your call. But a confession inside a family carries more risk than most: she is left holding something she must either keep from your brother or tell him. You are potentially better off sending a sealed IF, which says everything and asks nothing until you are both single.
Is it normal to be attracted to my sister’s husband?
Yes. A brother-in-law can be part of your life for decades, and that kind of familiarity builds closeness. It is also manageable: sleep somewhere else on family visits, keep your time with him in company and stay out of their arguments. And if you want it said, a sealed IF can say it without touching their marriage, so that if the timing ever lines up, you are there.
How do I stop having feelings for my sister’s boyfriend or brother’s girlfriend?
You cannot switch a feeling off, and you have nothing to be ashamed of. Give it less to feed on: see them in groups, mute them online, spend time with your sibling on their own and fill your own life. Writing it in an IF can be what finally lets it rest. Measure progress in family gatherings, not days: each should feel a little easier than the last.
Can I date my sister’s ex?
Sometimes, eventually. It is your call, but it goes far better with her honest blessing, asked for before anything starts. Give it a long time, and remember that if they share children, the ex stays family for life. Some sisters will never be comfortable with it, and that is their right. If the day comes and you are both single on IF, you can send a Like, and if it is accepted, a chat opens.
Right person, wrong time? Wait for them on IF, and let every family dinner carry on with your feelings safely sealed.