How can I help my niece and nephew?

Full disclosure, not a parent but until recently when my sister moved out I took Care ofmy two year old niece and three year old nephew daily. I didn't have a job for months and their parents don't watch them so my days usually revolved around them. when they moved in with my mother, I got an overnight job so I don't see them as often. Gonna vent below.I love my sister, but she's terrible to my niece. She treats her badly and clearly likes my nephew better. At their new place, when my mother isn't there my niece sleeps on the floor in the living room because my sister won't let her sleep in the bed with her and my nephew. She buys snacks for him but not for her and she bathes him more often than she bathes her. They both have lice and my niece's hair is Tangled and a mess.The father is addicted to medical marijuana and is high 24/7. Yesterday he was so high he fell asleep while they got out of the house and played alone in the front yard. He has anger issues and hit them hard enough to bruise when no one's around. He has no jobs, has been entire first year of their lives in jail, and barely watches or care for them because he spends all his money on weed. We literally found them playing with his bag and my nephew already knows what weed and high are.They're both still in diapers, they have no books no coloring toys, my nephew's barely learning to pee in the toilet, and they spend the majority of the day watching tv. Neither eat breakfast or dinner unless my mother is there. I've wanted to report them to cps so bad but I'm scared they'll run off to his hometown again with his ghetto family. My nephew spent his first year and a half there until their father was finally put in jail. When they came to live with us, my nephew was bones. He was too scared to look anyone in the eye and it took him a month until he would talk. My niece didn't go to the doctor until she was over 1. They had to give her did shots at once.So I don't know how to handle this anymore. I'm scared they're gonna run off when their dad falls asleep or hurt themselves. I don't want to call the cops on the drugs because I don't want my mother to get in trouble for trying to keep the babies safe. I dont want them to run off if cops gets called or lose the kids. We've been hoping my sister would mature or leave him but they have both gotten worse. I don't know how to go forward from here. I want to buy them books and try to potty train them but it's hard because I don't see them often anymore. I bathe them and comb their hair because I know my mother is the only other one to do it. I cook them fruits and vegetables and give them water. They drink nothing but soda at the house and their teeth are bad because they don't brush.Sorry for the ramble and all the errors. Got a new phone and I'm half asleep watching them because their dad went wherever at the last second so I'm not getting sleep before work. Not even sure what the question is, I just don't know how to help without losing them... via /r/Parenting http://ift.tt/2d9KRIc