Some quick thoughts, as my family always lived like this, and we still do, although the players have changed as the great-grands and grands have died off. We are actually in two separate houses around the corner from each other, as we could not find a two-family in this area big enough for seven people and guests/live-in health aides when needed, etc. Historically, the family has either lived in duplexes, or a property that had two houses on one lot. I agree with everybody who recommends separate utilities. Also, you need to have good boundaries in terms of not getting too involved with personal "stuff." At the moment, my daughter and s-i-l are having some marriage concerns, and I don't touch any of that with a bargepole. I also find that as a grandma myself now taking care of a very fragile, senile grandpa, and also having some grandchildren responsibilities, I have to be very firm in what I can and can't do, or I get taken advantage of big time in terms of childcare. (Not that I don't love them, but hey, I'm only one person, and being an Alzheimers caregiver keeps me busy just doing that.) Having said that, it is a godsend to have a son-in-law who, if he's around, will snowblow the drive or help pick up grandpa if he falls and I can't get him up. There is a built-in social life in terms of holidays, birthdays, etc. And between both our houses, we can accommodate other sick, elderly relatives if we need to help out, or our preacher cousin who in bad weather needs to stay near her church, which is around the corner from us. So, a mixed bag, kind of like the Waltons of the New Millennium, just not so goody-goody and schmaltzy.