Well as a woman who used to do the same I d just be mean to my brothers n nicer to my sister.. I would say because of many things like. Jealousy and feeling like he gets to do more things specialy in Egypt where it's kinda strict for girls.. Ofcourse I realize that later on in life and I regret it deeply eventho my bro turned out to be a great man.

I remember how I d make him play as the supermarket guy and just let him sit far in the corner waiting for us to come buy things while me and my sis playing with doll house.. N we never rly play with him. He was the only boy too.. .. .. Damn I almost cry now..xD

As younger humans we have so many emotions and feelings... N we r still young to understand them so we let them rule our body and act accordingly..

Good thing is that girls are kind inside. No matter what she deeply loves u so much and would do anything for u.. I d suggest u talk to her. (even over n over) . With ur true feelings.. Hey sister I feel sad Cuz u do this n that. and I love u.. Please be nice to me. I care alot about what u think of me..

It's just her ways of dealing with things in her life. Getting them out on people who r close