Am I part of child abuse??

Please no rude comments as I've done everything I can to help this little girl. I've been roommates with this girl for a little over 6 months. We both have kids. I have an 7 y/o girl and she has 2 girls as well, one is 13 y/o and the other one is 6 y/o. Right from the back I noticed we both have different ways in raising our kids. I'm completely open and loving with my daughter and I'm all about emotional intelligence. She has quite a few responsibilities for her young age. Walking the dog, feeding the dog, making her bed before school, folding her own clothes after I've washed them, also leaving her outfit ready for the next day etc. Tasks that are beneficial for her and help with her independence. Well, the other 2 girls are a whole different story... there's dog shit and pee all over the leaving room bc they won't walk their dog, there's no bed time, there's no lunch time, pretty much there's no rules since my roommate works from 9am to 1 or 2am 4 days a week. The other 3 days she doesn't work but she goes out and leaves the kids alone... which starts all the drama! The 13 y/o constantly abuses the 6 y/o! Physically and verbally, she hits her when she's annoyed she hits her if she doesn't listen she yells at her as the only way of communication they know (actually they both yell at each other), she can't say a word to her without cussing her out, she locks her out the room (2 bd apt), and she won't feed her unless the mother leaves something ready for them (not always the case). The other day she literally told her she wasn't going to feed her bc she was going to eat all the food and... she eat all the food! I had to buy something for the 6y/o. I try to involve her in my activities as much as I can, but at the end of the day. I need my space and so does my daughter. I work from home I see those girls more than their mother. At this point I really don't know if I'm doing her some bad or good, example, I bought Chinese for my girl, she asked for some, my daughter shared 3 wings and rice with her. Next thing you know the 13y/o gets home with lunch their mom had just bought and tells her "you eat already? I'm eating your food then" so I told her -we only shared 3 wings and rice she probably stills hungry. This girl completely ignored me and then proceeded to eat both lunches. I had to make something for her. I spoke to the mom already, her first reaction was "okay?! What you want me to do? Kick her out of the house???!!!!", second time, nothing changed, third time... nothing has changed, several other times and she just tells me to ignore her bc is part of the age. One day I straight up asked the 13 y/o why was she so dedicated to bully the person she was supposed to care for? She just rolled her eyes... I think that's the answer! She doesn't want to take care of her! Which is understandable but doesn't justify the abuse. My roommate has had about 8 abortions, and on the last one, she had to get rid of the baby bc the father is a married man whom she's deeply in love with, so he gave her a dog instead. So when she finally gets home and the little girl is happy to see her her response is "get the fuck out of here, fucking annoying little girl with no neck! Don't you see I'm playing with my baby???" While she's kissing the dog. Sometimes the little girl just apologizes and she leaves some other times she insists and then the mother hits her in the shoulder or back pushing her away. The mother has seen first hand how the bully threats the 6y/o and instead of doing something she adds to it. She has leave her 4 times by herself since the school year started (2nd week of August) bc the girl was sick. Like I said, I work from home so I'm always with her, but I also have to pick up and drop off my daughter to school, I can't bring her with me. If she drinks too much with her boyfriend then she doesn't get home and she leaves them by themselves (remember I work from home, I'm doing the feeding) until the next day when she gets off work. The little one is morbidly obese, that's why she calls her "no neck little girl" every now and then she has a bruise or a scratch but she's never had a black eye or something. Except for the one time the dog bit her on the nose and the mom especifically told her not to mention anything about it to the teacher at school otherwise they were going to take the dog away and mom was going to be really sad, and that being bitten was was her fault for wanting to play with her dog. I've told the 13 y/o that I'm calling CPS or the cops on her at least 3 times and she just laughs it off... part of me wants to do it, part of me is scared the girl might end up with a worse family. I'm not staying until the end of my lease, I can't take this anymore. Its literally making me have nightmares about the little girl committimg suicide, I'm leaving by October and I already told the roommate why I'm leaving for the million time she just told me that I've been intrusive, that I think my mother skills are better than hers and that if something happens I can just say I didn't see or hear anything but I like to take things out of proportion. I'm pretty sure that if something happens the police will held me accountable for being the only adult present with them at all times. I know that no one is going to care for the little one once I'm gone... I really don't know if I should call CPS... since there's no very much physical proof of what I can report.