Very true, with Mary I can definitely see them growing to hate each other and always wanting to outdo one another, and Mary encouraging that behavior. Also I can imagine her preferring Dean because Sam has demon blood in him and I don't think that's something that she'd be ok with, even though it's her fault.
True, she has a very twisted view on family. To her family is disposable, and listen I'm all for kicking people out of your life even if their family sometimes, but she seems to discard them like if they were tissues. If you don't bring her any use you're on your own.