I'm halfway through my 8th year of not speaking to my mother. The proximate cause was a flame letter she wrote me one month after my wedding, where she said I was arrogant (even my camp counsellors said so) and a coward and that I'd raised victimization to a high art. She concluded that (my paraphrase) unless I was able to keep my feelings to myself and keep that bitch of a wife in line, she could no longer be in my life. The break had been building for 33 years. Since then, she's sent me occasional birthday cards (including one 2 weeks after she sent me that letter, including a small packet of some fruity tea), and a hear-you're-having-a-hard-time-if-you-want-to-talk-about-it-give-me-a-call card, after I was laid off just before I could quit my job in 2001. Oh, and an email asking if she could print out one of my photographs from my website, rather than purchase one; had to save her money for her trips to Russia and Spain, I guess. Not that I'm bitter.
I've known people who've been really badly mistreated by their parents or siblings who continue to remain engaged with them, because they're "family". I've also seen so many people who treat their friends far better, with far more care and consideration, than they do their family. I've been guilty of this myself (though in my defense -- and there's always a defense -- I tend to be a spotty communicator with everyone, but if a member of my family shows up, I do drop / rearrange everything to spend whatever time they're willing to spare). There's this attitude that, to paraphrase Robert Frost, "[family] is...where, when you have to go there, they have to take you in." But family should be so much more than this.
Family should be your bedrock. Family should be the people with whom you'd most like to be spending time. There should be no question or worry that you might be intruding on them if you want to visit, no fear that they might not want to spend time with you; visiting family shouldn't require an invitation, engraved or otherwise. Of course, lives are what they are, and some times are better than others, but if someone's too busy to see you, they should be able to say so, without fear you'll read something into it. Family should be trust, and care, and value, and interdependence, and honesty, and heritage, and loyalty, and support. It is not shared genes, or obligation (though as in marriage, that's sometimes all you have to get you through the inevitable rough spots; it's all the rest of it that keeps those rough spots short).
My first falling-out with my twin came when we were 17. We'd been growing apart through middle school and high school, and I couldn't understand why; I even wrote a poem about it. When we finally talked about it, in the bedroom we shared in Switzerland, with the lights out, I said I felt like I didn't know him well but I wanted to know him better -- I think I said our older brother had said that my twin had changed and I should see who he was now -- and he said he didn't want to know me better. (To give his side of it, as I understand it, he felt that I hadn't given a damn about him and to hell with me if I was just now, a couple months before we went to opposite coasts to go to college, interested in knowing him.) He could give me no reason as to why and didn't understand why he needed one: he didn't need one to disregard anyone he met through other means than sharing a uterus. I said then what I still believe: family is special, and you'd better have a damn good reason for disassociating from someone in your family. But when most of what you get from someone in your family is pain and sadness, withdrawal is important / good / appropriate; family doesn't trump self-protection.
Family is as family does. The best of friendships evolve to be family. In my life, I distinguish between "relatives" and "family". Relatives have something of a claim on me, more than a random person on the street, and they remain in that camp until they demonstrate they should be in "family" or "disowned". For family, I will sacrifice / put up with a lot.