AITAH for not protecting my half sister from my stepbrother?

AITAH for not protecting my half sister from my stepbrother?

My mom married a dude whose son (15) has really bad anger and aggression issues. It’s so bad he was expelled from school twice since I (16) met him 5 years ago. He was taking meds and going to therapy which didn’t do shit to calm him down. A few times neighbors had to call the cops on my stepbrother because they could hear him next door and they were scared he might leave the house and start doing that shit outside. He never got arrested but he was placed in a psych hospital three times. CPS was involved a little but they never did shit from what I saw.

Then my mom and her husband had my half sister 3 years ago. Her birth made my stepbrother worse. He’s not his dad’s bio kid so that was something he couldn’t get over and it bothered him until I was removed from my mom’s house. He hated her more than he ever hated me and he hated me whenever his dad said a single thing to me. I don’t even like the dude and he didn’t take my dad’s place so there was zero reason for him to be jealous but he was.

Eventually I started calling CPS all the time. I wanted out of my mom’s house and I had grandparents willing to take me, but mom said she needed me at home and my half sister needed me. The only time she ever semi-considered it was when she asked my grandparents if they’d take my half sister too. They said no, because these are dad’s parents and not hers, and mom kept saying no after that as well.

CPS didn’t do shit for over a year. Every time they came out they left again and didn’t admit things needed to end. It took my stepbrother having a gun for them to remove me. My half sister was removed temporarily too and placed with foster parents but my mom and her husband got her back while I stayed with my grandparents.

When mom got my half sister back she told me I needed to move back in and protect my half sister. I told her if she wanted to protect her she should leave her husband and put her daughter first. Mom told me it was never that simple and she asked where my instinct to protect my baby sibling was. I told her job and her husband’s job to protect my half sister, not mine. Every time mom calls (and I need to answer and talk to her at least once a week) she brings it up and she tells me I shouldn’t be able to sleep at night and I should be more worried about my half sister. My mom won’t send her back into foster care and she’d still let my grandparents take her if they were willing but they’re not. So she keeps putting it on me.

AITAH? The reason I might be is the fact my half sister is back there doesn’t keep me awake at night and I don’t feel like it’s on me to protect her. But I know her parents won’t so maybe I should do more knowing that.