I think my baby sister is in danger

Long story short I have a 3 younger siblings but two of them are in their teens. One of them is only two years old. We were all verbally and physically abused by my father (who we lived with because my mother is on drugs) me and my other (teenage sister) moved out, I turned 18 and she found out she had a different dad! My brother (12) and two year old sister (different mom) however still live with him. My brother goes through a lot but my dad pretty much ignores him for the most part but he lives without heat or hot water due to the fact there is no gas in the house. He has other family he can stay with but claims he is fine. My baby sister on the other hand who is two is a wreck. She just got taken away from her mom because she was on meth. She would always come to my dads red all over her private’s from not being changed, she was always sick, always crying, just a miserable child. They recently gave temporary custody to my father and he works so I have agreed to keep her pretty much day and night, mostly to get her away from him. However I have rent and he wasn’t really paying me and when he did it wasn’t enough so I had to get a job. Now I leave her with my dad during the day and pick her up around 6:00pm she always cries and screams when I leave and is so happy when I pick her up. My dad watches her at my grandmas so that he can help him watch her, so I know that for the most part she is not to badly mistreated. But I have seen him hit her not like badly but I believe that anytime you hit a two year old it is inappropriate and he doesn’t do it for good reason. He screams at her and she is mentally behind for her age. She can’t talk, she is not even started potty training if you try to put her on the potty, bath her, or anything like that she starts screaming like she is being stabbed, if we go out and an adult talks to her she starts growling at them or crying and hiding behind me. That’s really only half of it but I don’t want to make this to long because I want people to read it and give me advice. Her mom might be getting custody back soon but I do not want to leave her with either of them. However I’m just now graduating high school, and I work at Wendy’s I cant afford a baby. And my apartment has roommates and I basically have to sneak her in I mean the apartment is like separated enough, to where she has plenty of room to play but it’s hard if she cries because I don’t want them hearing her and getting mad. I love her like she’s my daughter she literally refers to me as mom and it’s one of the few words that she knows. CPS is already involved due to her mother but my dad is really good about staying out of trouble with them and I don’t want to really tell CPS about what’s going on because I don’t think I have enough for them to take her but I also don’t want them to take her because she does have people that love her and foster care is a messy messy thing. My boyfriend says I need to stay out of it because I am to young for a baby and I have school, and he really doesn’t care for her that much. So what do I do?