I am trying to get my sister to go away and stop lifting weights with me.
Is there a reason why you don't want to workout with you sister?
I am trying to do weightlifting for a six pack. She only wants to ever work on her butt and thighs.
What you want to work on is going to be different than your sister's. Everyone has their own workout goals so you shouldn't tell your sister what to do because at the end of the day it is her body. By telling her to stop working out with you only makes you look controlling, and selfish and she will probably be very sad about it.
She is too fat and I'm making sure she doesn't turn out like my mom.
It is completely up to your sister to work out on what she wants to. How you feel about your sister is not really important in this scenario to be honest! You can always just go to the weight room at a different time as your sister instead.