Is it normal that I hate my gf's kid?

My gf has 2 kids one is 16 the other is 12The 12 year old is ok, when she is not around her sibling. But the 16year old I would love to clobber her right in her fat face! She is a fat 250 lb girl that dreams of being a 105 lb mean girl. She is a chicken shit starting fights with me, then using her mom to protect her when it becomes too much for her. She picks on her sister all the time, and when the younger punches her, she cries and tries to get the younger on in trouble. She is lazy, obnoxious, can't spell worth a hill of beans, and everything is "ghetto". And even though she has never had certain experience she, she knows what she is talking about and my gf backs her up. We were in a black neighborhood and all she could say is the n- word and all these people were ghetto, and she felt unsafe; even though it is a middle class neighborhood. She chews like a cow with her mouth open and smacks her lips, and then she has the nerve to make fun of other fat people. I wouldn't clobber and I am venting, we had bad week this week.