Because the only real family that we choose are our friends.
The rest is karma ;-)
Because we spend most of our time trying to be independent of each other.
Because they love each other and care enough to fight.
I know no matter how bad I behave with my family I will always have a family. I have screamed at my sister and vice vera over little/big things in my life but still she my sister and will talk to me and things like that. I have disappointed my dad but he still love me to no ends. Family fights can also show how much bondage we have because after all that we still a family. I can look back and say they really stuck by me even after all the fights!
Because they don't know where one person stops and the other person begins.
Yeah Emptysoul30 is right. Siblings and parents all fight no matter what. Whether it is between the siblings themselves or them and the parents. We all do it and it helps us for when we are older. Because all that fighting against each other for all them years will end up helping when you need to fight for each other later in life.....