Why is it when the younger sister is taller and stronger than her older brother?

It might just be jealousy of power. There's a pre-judgment that the older son or daughter has more power over there younger siblings. In my opinion this is one of the roots of sibling rivalries, by saying that a child has power there own brother or sister makes the child build an ego of power over most of the thing he or she'll do in the future.

maybe because there is one of u and 2 of them and because of that maybe they think they can over power because of defeat of number or maybe because like the first answer their mad because u hold power over them and can tell them what to do and when to do it within reason