I think we are full of love and hate. In family relationships, particularly with siblings, aggression and difficulties precede affection and love. At first this person seems like a rival and then, later on, you can have kinds of feelings. But there is a great deal of it built in. It makes sense: evolution requires us to be the one who survives, to be the one who prevails. But I think what screws up siblings more than anything else are parents.