Yea - I agree with what Tonberry mentioned regarding blood. Really, in the bigger scheme of things, it doesn't automatically mean a thing. Funny, I've had this same conversation with another close friend of mine who was struggling over why blood family just wasn't connecting (nothing to do with poly etc).
I guess what we need to remember in the end is that we are all just individual spirits drifting along our own paths. Along the way, people will come in and out of our lives at various times, for various reasons. In reality - blood family are just more of those individuals. The key seems to be perceiving the reasons - NOT whether their appearance was of our own design.
I think this phenomenon brings a lot of people to poly. At first many think it's about sex, or filling in missing pieces, or getting needs met etc etc. But to me it just feels that something larger and more mysterious is at work. And my natural curiosity is enough to keep me fully engaged
Seems I'm constantly living in a "welllllllll - what do we have HERE mode !".