wat i have seen is dat mostly in families parents dont have favourite child. They give importance to all of them.and try to help them in different manner because they know each of their child is individually different. So they treat them differently.As far as favourite is concerned this is decided on their habits.
I know a family who haz two sons. they love their children a lot but they usually express it to their younger son but not to the older which makes him feel i am not wanted. We know this is true but this is also true that many children have fav mother or father too