To anyone who's yet to have an in-law: My best advice is this: Just because you may (start out) feel(ing) warm and fuzzy toward them all and like you are their brother/sister...Don't assume they feel the same way. And then, don't cry on their shoulder/confide in them/etc., etc. It can all hit the fan later in a really ugly way. Not everyone has kindness, decency or compassion. Not everyone understands their ABCs for that matter. And some people just look for divisions and basically do not ackowledge their siblings' growing up and actually having a family themselves.
On the plus side if that happens your Christmas card list will be trimmed wayyyy down.