Am I overreacting by cutting off my Friend for something her sister had done to my sister?

I (16F) have been in a close knit friend group of three for the past few years. We are very open with each other and I thought we were all best friends. My sister Dahlia (fake name) (13F) was best friends with one of my friends (Karen 16F, also fake name) sister (Brittany, 13F) so we all usually hung out together at my house.

So what happened was, my sister who normally shares every little detail of her day with me suddenly became very closed about two months back. No matter how much I asked her to tell me about her day or anything she doesn’t talk and she started spending her time on her bed or with her phone, avoiding us (me and my parents) entirely. She would have random outbursts where she cries and just stays staring blankly with no explanation whatsoever for what has happened. We all just assumed she was having a hard time with studies

The other day, she was actually conversing with me and all of a sudden she started to say that Brittany might have a crush on her (mind you it’s against our religion for same sex couples to be together) and first I just brushed it off as something fun, but the more she told me details the more it piqued my interest.

Dahlia told how Brittany has been showing signs of emotional manipulation by telling her things like no one else will understand her and that others will judge her. And she has been often complimenting her with sexual undertones regarding her voice, smell and her body. Even though she told me about these, I didn’t take it so seriously until she burst out crying and told that Brittany had tried to pin her to the wall and get physical with Dahlia. And that point i knew i had to take action.

Like i said before, Karen who is Brittany’s sister was one of my best friends so my first instinct was to tell her what was happening and to put a stop to it immediately. I texted her and told to meet up so that we can discuss this.

GUESS WHAT?? SHE KNEW ALL ALONG. When i went and told her the first words out of her mouth was “I’ve known for a while, i just didn’t want to embarrass YOU” WHAT DOES SHE MEAN BY THIS? She just allowed her sister to keep SAing my sister??.?

Anyway, while we talked, i find out that it wasnt just the one time, but twice that we know of. One time it was Brittany literally forcefully having her way with my sister. WHICH MIND YOU, KAREN KNEW OF FROM THE DAY IT SELF!!!! Naturally i lashed out at her and told her that she has betrayed me and lost my trust on both her and her sister, and what she did is something that i will never forgive her for. She showed no remorse and actually tried to pin the blame on my sister saying that it was consensual and that they both did it together and that Dahlia did it to Brittany too.

Owing to all this, i have cut Karen off and made it very clear to her that i want nothing to do with anyone of her family. However, due to her careless behaviour I am having second thoughts on this. Im not sure if i’m right for cutting her off or not. it doesn’t help that the third friend of our group is defending Karen side on this (the fact that Karen hid this whole thing from me) So, AITA for cutting off my friend because her teenage sister assaulted my teenage sister?