sounds like my 9 yo..
deliberatley with hold toys from her or slams his door in her face or just plain ignores her..
or tease her with food or just push her away or be mean

typical sibling behaviour.

what i do is every time he does it i say now stop and think about what you did and how you would feel if someone did that to you? and if he says i wouldnt care. i wait 10 min and do it to him he soon realises what it feels like.. prob not the best course of action but it works for a few hours. and now all i have to do is say stop and think about how you would feel or react and he does now.. only took 12 months though.. sorry..

its ok they do learn to stand up for themselves, DD is doing that now and teases him and with hold toys from him lol and i just say well you do it to her she is only going to treat you the way you treat her if you dont like it change your actions..