AITA for telling my brother it's not my problem that he can't keep his hands to himself?
So I (17f) am a triplet (a sister and a brother) and also have 4 other siblings (4f, 9m, 12f, 15m,) So our family is very big. Me and my sister are extremely close and my brother has always been jealous of us. Our other younger brothers are also very close and same with my younger sisters (before the youngest was born and for 2 years after that twin brother and younger sister were close). Since all of us each have a close sibling my brother got anger issues due to feeling isolated (All of us tried to include him in our activities but he refused). He once stole a bag of chips from the store when we were 15 and later blamed it on twin sister telling him to. Later I heard him admit that it was so that I would be mad at her because I have had very bad situations associated with stealing.
Another time, he copied my hand writing, wrote a whole journal about how I hate my twin sister and wanted her to die. Then he kept it on her desk. Luckily twin didn't believe it since I journal online. He's done similar things to the others as well.
So a few days back we had our final exams. He has a gf so he was holding hands with her in the hallways but then, he put his hand under her shirt (poor girl was shaking) then unclipped her bra, made sure everyone was looking, turned to me and said "I completed the dare should I stop now?" Long story short everyone except my other siblings (they knew that he had done this stuff before) believed his version.
So me and him were arguing in front of my parents because he kept insisting I was the one who told him and I told him "it's not my problem that he can't keep his hands to himself". My parents pulled me to another room and slapped me as I'm the reason he did this. Brother laughed at the red mark I got.
I'm being blamed and gaslighted by everyone including his gf (I don't blame her though). The only thing I'm happy about is that my twin sister is staying by my side. Also one thing to note is that twin sister has been r@ped before so I think this was another plot to get twin sister away from me.
Edit: I'll address some things that were in the comments.
Camera Footage: Our school is getting the cameras fixed atm. Which means there's no cameras anywhere. Brother realized that he could do something like this and get away with it since there's no proof. Only witnesses.
CPS: My younger brother has called them before multiple times but they don't do anything
Police: Gf went to police but according to what I heard they're looking into it but not doing a lot because there's technically no proof. Only witnesses and word of everyone.
I don't really know how a lot of these things work because I just heard stuff (from highschoolers and idk if I should trust what I hear from them) but the only good thing is that I'm going away for college next year which means an excuse to get far away from these people.