it was interesting reading the previous responses, i'm surprised by how many didn't meet each others families until right before or right after being married. doing something like that never would have occurred to me. i know that a military relationship makes something like this more common, but i would feel weird having things work out that way.
my DH and i have been together for 9 years, married for 5, and our son is 7. he met my mom about a month after we started dating when she came in to PGH for work, met my sister and our now BIL and my stepdad about a month after that, then the whole family, and i mean the WHOLE 35 plus person immediate-extended family, at Christmas that year.
i met his brother and our now SIL about a week after we started dating. his parents and younger brothers were on vacation, so i met them about 2 weeks later. the way it was done was unexpected, but it didn't have a negative effect. i LOVED my MIL, she was an amazing women. unfortunately, she passed away unexpectedly at the end of 2003, so our relationship was not a long one, but it was great. my DH is so similar to her in the way he treats those he cares deeply about. i'm still not as close to my FIL, but that's just because he is very quiet and reserved, and reminds me tons of my own dad, whom i'm also not overly close with.
i am pretty close to my BIL and SIL. they live down the street from us, and their oldest and our son are only a year a part, attend the same school, and their girls and our son go the same daycare. it has taken some time to get this close for me, but my DH and BIL were always close, they are only 368 days apart in age (DH is the older one). my SIL and I actually just trained for and ran a half marathon together, so that really brought us closer.
i am also pretty close to my 3 younger BILs (nearly 16, nearly 17, and recently 18). they kind of grew up with me, and they are our defacto babysitters. they spend the night with us frequently (we live in the same school district they do), come over to help around the house or just to hang out. i live 3 hours from 99% of my family, so i'm so glad that i have the relationships i do with my husband's family. my DH is as close as he can be with my family, given the distance and his work schedule that doesn't really allow for weekend visits. however, he gets along great with all of them.
now, he also has an enormous extended family, and they all live not too far away either, so we spend a decent amount of time with them. i'm pretty close to a number of them, particularly those who are the same age range we are, and don't have any problems getting along with anyone else. i'm definently lucky in that aspect.