Why does little brothers think their smarter then older sis's?

I would say the answer is because they are in fact older.

most families would even rely on the older sibling to babysit. and because of this they come to think of themselves as the boss. Sadly in some cases the older sibling does indeed become the boss enforced by the parents. which really should not be unless there is a significant age and maturity difference. if the older sibling is only a few years older they really shouldn't be an authority over the other (except in babysitting situations) especially if the older sibling isn't all that much more mature than the younger.

Hope this Helps.