I think it depends completely on the people involved, and how adaptable they are.

I've seen some situations where a son/daughter never seems to get recognised and respected as an adult, and as a husband/wife/parent - and so there's a strain and sometimes a deterioration in the relationship ...

Other times, one family is a closer knit family than the other, or they're more physically affectionate, or generous, or already have regular meet-ups and so those dynamics continue on into the marriage, and so while the couple still meet up with family A every week/month/whatever, or chat and email regularly, the other family is only seen for special occasions and accordingly, that relationship isn't as rich - because they aren't as aware of what is going on with the family.

We are starting to see that with DS's relationship with his grandparents ... we see my family every Sunday for lunch, and then also for birthdays (between the whole family, that's 11 of us per annum!) and then special lunches for Christmas, Easter, Mothers' Day and Fathers' Day ... and my family are big on presents - it might just be a $10 toy, or home made cookies - it's not about the money - but it's a real display of love. My PIL on the other hand almost never invite us around, and they've seen DS maybe 10 times ... they love him very much, but there's no structure to the relationship, and so it's not clear whether DS actually recognises them ... and while they are generous (eg: if we were in financial trouble, I have no doubt they would assist, or let us move in or whatever) they don't seem to be "gift" people. If we asked, I'm sure we would receive ... but should we have to ask?

All this means that I am a little concerned that as DS gets older, he will heavily favour my family, because he sees them more, and they are so much more expressive about their love.

As an adult, I know that my in laws adore DS just as much ... but in their own way ...