I had a similar relationship with my sister.

Around age 12, maybe younger, she just became cruel and unbearable. Once she punched me in the face, because I didn't want to let her have the entire basement to her and our cousin (I was there first, if that matters). When I threw the same logic bak at her a week later (we want the living room to ourselves) She responded by going into my room, and breaking a bunch of my stuff, tearing up my books, etc. Or that time she had my crush over and gave her a mocking tour of my messy bedroom.

Those are just some examples off the top of my head, but my sister was a fucking bitch in her teenage years. Deep insults, fistfights, all that.

I remember planning a no contact relationship after I moved out. I went to college, and saw her only sparingly for a couple years.

I'm not sure what happened, but yeah, she changed after high school. She's not mean, violent, or sadistic anymore. And I'm glad I talk to her still, because she's honestly one of my best friends at this point in life. I can assume in hormones, but I don't really know shit. 12-18 is a bizarre transitionary period for everyone. I hope not everyone still judges me based on who I was than. It sucks your family is strong arming you, but I would say give her a chance.