My friend and I pinned my older sister's boyfriend while we were playfighting. Why is she SOOO mad at me?

She has her boyfriend from college over and it was mostly her idea to start with. She thought it was funny. But after we had him down for about 5 minutes, she suddenly changed her attitude and now she's really pissed. wtf...
My friend and I pinned my older sister's boyfriend while we were playfighting. Why is she SOOO mad at me?
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