I think that a person's family says a lot about who they are, and how your relationship with them will go, whether it's romantic or not. Families are a peek into your future with that person, if they have a rocky family life, chances are that your relationship with that person will be rocky as well. But if they have a warm, welcoming family, that could give you an amazing relationship with that person.
This happened to me right before my eyes. I had a first love in high school who, of course, I saw myself marrying. But when left alone with his family, I felt uncomfortable. I always felt like I couldn't be myself and that I had to always be on guard for questions or conversations. As expected, that relationship ended. And when I met my current boyfriend, it was a whole new world.
I have the most amazing relationship with my boyfriend, but also with his mother and brother. I can sleep over his house when he's not there and have the best time. I spend more time with his mom and brother than I do with him (#marinegirlfriendprobs). I am so completely comfortable with his family and that means the world to me. Families play a huge role in a relationship and I am so blessed to have such a great relationship with his family.