AITAH FOR SIDING WITH MY SISTER..??

In order for you guys to understand, I'll have to start from the beginning, and it's a long story, so I apologize in advance.

My older sister got pregnant at 13 and had her baby at 14(boy) and then pregnant again at 14 and had her baby at 15(girl). When she had my nephew she tried her best to be the best mom she could be, but once she had my niece it was like if she was a mistake, when my niece got a little bit bigger my sister would hit her for little things and sometimes call her out of her name. It got to the point where one day my niece did something she wasn't supposed to and my sister was hitting her with the belt and the belt buckle ended up hitting my niece on her low lip,she was bleeding alot and if my sister would of hit her a little bit harder her lip would of been cut in half. My second sister and brother and I got into an argument with her and told her that she should stop being that way, because once she got older she would hold a grudge and be disrespectful towards her but she didn't listen. Going forward a couple of years my sister got married and had three more children. When they would make a mess my sister would put my niece to pick up the mess, only her no one else. She would also make her sweep and sometimes wash dishes, mind you she wasn't even close to 10 yet. My sister was married for almost 10 years, but after a while she got depressed and instead of getting the help she needed she leaned on drugs (pills/alcohol). That destroyed her marriage to the point where her and her husband got into an argument, and it turned physical. She called the cops and he got deported back to his country. She lost everything, she was left with nothing and she had no job so her only choice was to go back to my dad's house ( which i lived in cuz I wasn't an adult yet) and my dad helped her as much as he could. Once she moved in, she stopped caring for my nieces/nephews. She would leave them alone at night without telling anyone and not appear until the next day or until my dad or I would start calling her. That happened constantly, and it was turning into a problem where now my dad, second to older sister and I were starting to stress because dad had to work, I had to go to school and second to oldest sister had to stop working just to see what we could do to not leave them by their self. One thing my dad has is a lot of patience but he had enough of my sister's actions so he decided to call child protective services (cps).

Once cps got involved they informed her that she would need to go to rehab for 90 days and she agreed. Older nephew went with his dad, nephew and niece stayed with us and the two little ones went with my sister. Due to me going to school and dad traveling to Mexico and back because he was on the process of getting married he decided to take my niece and nephew with him so that his wife could take care of them. IT WAS THE WROST DECISION EVER!!!!!

When my sister finished her rehab and she was able to get her children back, she apologized for everything, for all the hurt and damage she had caused, and she actually was better. She was doing her best. She got her self a job but due to that job taking up all of her time she chose to take my nephews and nieces to my mom's, my mom ended up going back to her country (Mexico). While they were with their grandma, she asked them if they went through anything traumatic. My oldest niece informed my mom that she got touched by my dad's wife son. When she said that my mom called my sister and second to oldest sister to inform them of what she's been told.

Just FYI my dad brought his wife, her son and daughter to the USA. So when my sister was informed of the news she went crazy to the point where she broke his car windows, had him jumped multiple times and even made a police report. At the moment he's doing 10+ years in prison for what he did.

Moving forward to now because that happened almost 8 years ago. My niece is 16 now and holds a huge grudge against my sister. Recently my sister and niece got into an argument. My niece is yelling at my sister and telling her that it was her fault that she got touched. That she don't love nor want to live her. That she hopes she rots in hell, and that she wants the worst for her. My sister has had alot of patience with my niece but that was the last straw for her. My niece has even gotten physical with my sister and her sisters. After the argument my niece snuck out and ran away for a day. She appeared the next day at my second to older sister house. My sister was informed and my second sister told her to let her sleep with her for a couple of days. That turned into a huge argument where my second sister told my sister that that's what happens for treating her like a servant and for treating her like if she doesn't matters and that my nieces actions are justified because of what happened to her when she was little. My older sister stopped talking to my second sister. I don't agree with my nieces actions due to my sister trying multiple times to make amends with her, I also don't agree with my niece acting the way that she is and always bring up that she's that way because of what happened to her when she was little. I do feel bad and have a little guilt because maybe if I would of taken care of her that would of never happened, but I can't change the past.

I am open to all opinions, because I've thinking about this for a couple of days now