Do you try to become friends with your sibling's SO? Do you try to become friends with your SO's siblings?

My sister's boyfriends are extremely friendly with me whenever we meet up. I think (with the exception of one) that they're just being nice to me because I'm their girlfriend's brother, but it's possible that they are actually friendly to me because it's a pretty good idea to be your brother-in-law's friend. I know for a fact that my sister and girlfriend are friends (they met after we started dating). So I know that my sister actually wants to be friends with whoever will become her sister-in-law. I have met my girlfriend's sister a few times and she seems cool, but it's difficult to know if we'd be able to become friends since she has to deal with 4-6 kids (she has 4 from her first marriage, her husband has 2 from his) and a pretty demanding job as a paralegal.

Do you try to become friends with your sibling's SO? Do you try to become friends with your SO's siblings?
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