Family matters

Recently I have had several little things happen which have got me thinking about the concept of "family" and what it means.
I have had to come to terms with one relative shocking everyone else with some "bad" behaviour.  I've also had the fun of a visit from my husband's much younger brother -- along with his skateboarding and youthful innocence.
All this has been preceded with the acquisition of four step-siblings and an "evil" step mother in the last few years.  My step sister and I have had the luck of having babies within hours of each other.  My stepmother had the drama of dealing with these dramatic events.
Next week I will be seeing an old school friend who is the closest to a sister that I ever had while growing up and is the guardian of my children, should anything happen to me.
Looking at all of this, I start to wonder about what makes a family?  Does it have to be through a genetic link, or can we adopt friends into our families?  What about those members of our families that misbehave?  The relative mentioned above has been "ungodfathered" by my cousin, following on from his behaviour.  Is this the best way to deal with him?  What about just being there to offer some support while he tries to figure out the mess he has landed himself in?
I know that I would walk through fire for my "adopted" sister, and that she is likely to do the same.  I am not sure I can feel confident of this in some people who share my genes.....
I look at my two children and how Dragon has embraced his sudden new clutch of cousins and how Bumblebee loves playing with her "twin", and I think back to a feud between my cousin and I that has been going on for almost two decades, and which I still have no idea as to the cause.  I was raised to put family first and that supporting one another is vital to maintaining close ties.  My grandmothers were the matriarchs who saw to this.  Since they died, the ties have come undone rapidly and I see a close know family rapidly becoming strangers.
So now I ask, given that the people I count on to be there when my immediate family can't, are not related to me by blood, is blood really thicker than water?