Girls, How do I tell my girlfriend that I am not comfortable wrestling her sister?

Anonymous
My girlfriend has a high school aged sister who hangs out with friends at my girlfriend's parent's house. I am there often to see my girlfriend also. Sometimes we are all in the living room area at the same time. I would pick on her sister and her sister's friends sometimes, that was all fun.
Lately, her sister and friends have become a little more confrontational and it has evolved into wrestling. When there are 3 or more of them, they have even succeeded in pinning me / teasing me (they are athletes, stronger than I expected), which my girlfriend finds hilarious.
Problem is, while it is all just fun and nothing more, I've found that it kind of turns me on. I have tried to subtly avoid it, but my girlfriend teases me about me being afraid to lose, and encourages them to continue.
I can't tell her the real reason I need it to stop! And I haven't found a way to avoid it. It's getting to the point that they just assume we are going to wrestle like this when we are all together. How do I get this to stop?
Updates
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Somebody from this site gave me the idea to fake back pain. I am thinking about trying this next time. I have some mixed feelings about the idea, but maybe worth a try?
Girls, How do I tell my girlfriend that I am not comfortable wrestling her sister?
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