Is it a revenge or feeling thing?

So, right. I'll just talk about the topic. First thing first, my sister always bully me as she doesn't know what the feeling is to get bullied. Just now (not long ago) I was having dinner. My sis was in the living room watching tv and eating her dinner. My cousin (who always protect me from my sister) came back. I was so relieved that I didn't really have to worry so much that i would get hit. So, he came back and saw my sister. I think he doesn't like her to watch tv and eat at the same time, so, he told my sister to eat at the kitchen. From previous experience, she "ok! I'll go" face. then she came to the kitchen. my sis was mumbling something bad bout him so my cousin overheard and he came to the kitchen and like ask her what did she say. she, of course scared la, so didn't say anything. She complained to my father. then i wonder hmm, daddy's baby. Then, my cousin came in to take water and he pat my sis head say, eat ah eat somemore. Of course, there's nothing wrong with that. Out of anger, my sis ran into the toilet and bang the door "BANG" Then she cry lor... I was wondering what's wrong with her? she keeps bullying me but she doesn't like people to bully her. Sigh... She don't know how does it feel to be bullied. so is this a revenge for me or a lesson to her? I guess its a revenge to me and maybe nothing to her. Let me just tell u, the next day, you'll see her up and about bullying everybody. and she'll take it like nothing has happen.