Is it a good idea to date the relatives of your family members’ partners?

What are overall thoughts on dating relatives of your family members’ partners? Is it something that can work it out, or will it always crash and burn? The reason why I’m \[21m\] asking this very specific question is that I might be falling in love with the sister \[21f\] of my sister’s \[27f\] fiancé \[26m\]. So far, nothing has really happened between us, though she is very friendly to me (more on that later).

Even before my sister and her fiancé started dating, I shared a few anthropology and archeology classes with my future BIL’s sister. However, she was just another person in the background prior to our siblings getting together, though I did find her quite attractive.

It was only when my sister and her brother hooked up that we actually became acquainted. Our siblings often brought us along when they went out on trips and activities. At first, we only really interacted when our siblings were around, but lately we’ve been talking more and more on our own. We share the same interests in both anthropology and world issues, and we have long discussions about them.

My future BIL’s sister goes out of her way to wave me down whenever she sees me walking around campus or the store she works at. She’s really quick to respond when I text her some videos or articles I found, and she’ll often send some of her own back. A few days ago, we were both hanging out at my sister and her brother’s apartment.

When my sister and her fiancé left to get the pizza they ordered for all of us, my future BIL’s sister got up from her chair, and sat right next me on the couch. We started talking about the show that was on, and then she diverted the conversation towards my plans for next Saturday. I didn’t have much going on, as I have weekends off, and there wasn’t much assignments due that week either. So I told her I was free, and she followed up with a question about doing something together that day.

Having nothing better to do, I told her “yes.” She was overjoyed, and showered me with ideas like ice-staking, going out to the gym, bowling, mall, and even a few concerts. We couldn’t pin anything down that moment, and decided we’ll make up our minds later on the week over texts or calls.

While we were talking, I noticed that she also seemed to be probing my comfort zone. As she kept sliding closer to me, touching at my arms and shoulders “accidentally”, and even leaned her head on me for a few seconds. Perhaps that might be me reading too deeply into things, but I think she might be interested in me. However, she didn’t mention anything about dating, and I don’t want to risk overstepping myself. I’ve been laying my eyes on this girl for a few months now, and I’m afraid that I’m allowing my judgment to clouded by wishful thinking if I make such assumptions.

Until she is more overt with her intention, I’m going to presume that we’re going out as friends. One of my concerns with hypothetically dating her is that I might be encroaching on my sister’s relationship with her fiancee and his family. I’ve been told that it’s alway problematic to date a family member of your friends or your siblings’ friends.

I’ve known some people who’ve done this, and it’s quite mixed from what I’ve seen. One of my cousins \[22m\] got married to the aunt \[early twenties f\] of his younger sister’s \[12f\] friend \[11-13? m\] a few months ago. From what I’ve been told, it worked smoothly for them. Apparently both his sister and his wife’s nephew were very supportive and loved the idea of being part of the same family.

It was a very different story with one of my parents’ neighbors \[early thirties f\]. She had to deal with her teenage brother hooking up with her teenage stepdaughter behind her and her husband’s \[mid to late thirties m\] backs just before they got married. It got messy when her brother ended up impregnating her stepdaughter around the same time that she was pregnant with her husband’s child. I don’t know the full details of the situation, but the fights over handling it tore their families apart.

Some in the family wanted the child to be terminated, a few others sought to put it up for adoption, and a third group offered to raise the baby themselves. The neighbor’s brother and stepdaughter desperately pushed to get married, but that was shot down. They even tried running away together at one point. What finally happened was that the neighbor and her husband ended up taking custody of the baby after it was born, and helped the kids raise it alongside their child.

From both with what happened with my cousins and my parents’ neighbors, I’m really uncertain and conflicted if going out with her is a good idea or not. I haven’t talked to anyone about my crush on the sister of my sister’s fiancé. So I don’t know what anyone, especially my sister and her fiancé would think if I got together with her. Would it be a problem if I started seeing my future brother in law’s sister, and how do we prevent drama from happening like with my parents’ neighbors if this becomes something?

TL:DR: I’m just wondering how good of idea is it to start dating relatives of your family members’ partners, as I’m interested in the sister of my sister’s fiancé.