How can I deal with her Aggression?

My sister is one year younger to me, and some years back my mom, me and my sister used to live with my uncle, at that time i was 12 years, my uncle was a nasty man who was also fond of wrestling, so he would ask me and my sister to wrestle with each other in front him, and he would give money to winner, later he started asking us to try to take off each other's underwear, and who removed at first would be the winner , my sister was very aggressive and she would always try to take off my underwear to get the money, but i was a little stronger than to her

but my mom used to take my sister's side and she told her to grab my balls during wrestling and take off my underwear, and she started doing this, she used to grab my balls firmly and then she would pull down my clothes off, uncle and mom used to watch this wrestling, Now we have moved to another house but still my sister wrestle and grabs my balls, she either hit or squeeze it there like before and she don't listen me. how do i make her stop or get back to her? Now she thinks i am weaker to her and she bully me most of the times and my mom still takes her side ! how come i can teach her a lesson?
How can I deal with her Aggression?
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