Family Compounds

Here's the question: Have you ever known two families that liked each other? Think about the families of the two people who are your parents. Do they like each other? For me it is the "Spanish" ones who think their feces give off no odor and who think of the others as the "Dirty Indians". Yep, the same old story - they both sides were horrified their offspring wanted to marry each other. Here is what made me think of the question (and wonder if families hating each other is the exception or the rule). EssBee and I have been invited to the b-day party of my delicious neice, Leah (turning the ripe old age of THREE!). We love the girls (2 of them) and my brother and even his lovely wife. It's her family that I'm not too sure about. I often feel some kind of grating between her family and ours. I can't put my finger on it, all I know is that these gatherings are a collection of uncomfortable hellos, mandatory mingling (when the b-day girl opens her presents or blows out the candles) and then strictly segregated (with one family usually sitting inside and one sitting outside). The only other interaction is when we say goodbye. I've been to many weddings and the families seem to self-segregate, if they weren't placed apart for the good of all during the planning. I wonder if it's possible for families to integrate? I just wonder what that would look like? Feel like? Can anyone enlighten me? (and not remind me that rambling followed by a questions is a bit too Sex-In-The-City...)

p.s. The families of EssBee and I get along, but I would not call them integrated (mostly cuz of the anti-social behavior of mine).