My boyfriends sister is a little bitch and I can't deal with her much more?

Anonymous
My boyfriend, his brother and his sister are staying in the family home while their parents and youngest bother are over at the holiday home in Spain. My boyfriend is 18 and his brother is 16 and sister is 15.
The brother is no bother but the sister is a real cow. She argues with everyone constantly and cries when she doesn't get her own way. I watched her stamp her feet in temper yesterday because she wasn't getting to go meet her friends (it was half 10 at night and they stay a good bit out of town).
Then She was shouting and screaming at everyone today because someone ate her chocolate yogurt she had in the fridge and I lost my patience with her. I told her to grow up and that she was a little brat.
My boyfriend told me just to ignore her like he does when she starts acting up, but I can't. She's making me crazy. I said I would stay with him till his parents come home (still over a month to go) but I'm close to going home.
When I lost my patience with her today she started shouting at me and my boyfriend pulled her arm towards him (as she wouldn't listen to him) she went to hit him so he grabbed her other wrist and angrily spoke to her but didn't shout. He has the patience of a saint, I'm ready to slap her, but I know I can't. I don't know how much more I can deal with her. Is this normal behaviour?
My boyfriends sister is a little bitch and I can't deal with her much more?
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