Disciplining the Nephew

So my sister and I are 14 years apart. she's the eldest and im the youngest out of 4. She has a 8 year old that I had a big hand in raising until he was 3. I now have a 4 month old. so we both (along with our kids) have moved into our mothers home to get our credit scores up and both buy our own homes. shes been here for over 2 years (doesnt pay for anything except her and her sons expenses. doesnt have a sitter bc our mom babysits) and yet she still hasnt saved anything to buy a house. she likes to spend all her money on unnecessary things. I know she uses my mom for free housing and babysitting. however i want to be out of here asap. problem? my nephew (8yrs) is a spoiled brat. my sister and mom will let him rule the house and if he whines, he gets whatever he wants. he also isnt properly potty trained. he will pee everywhere and his mom wont clean it. when he talks back or wont listen I find myself yelling at him or making him do it.

**I wouldnt put my hands on him. ** BUT he needs some sort of discipline and its NOT my place to give it to him. my sister will yell at me for yelling at him for talking back. he wont brush his teeth, bathe, eat meals we make, he will stay up until he wants to go to bed, eat snacks all day, piss everywhere. ive gotten to a point where i just let him do whatever too, but its annoying when my mom or sister tell me my 4 month old is spoiled because I hold him longer after he falls asleep. if you put him in his crib as soon as he goes to sleep he will wake up. every time he wakes up they complain how spoiled he is and i wanna scream YOUR 8 YEAR OLD CANT EVEN PEE IN A TOILET LET ALONE SLEEP BY HIMSELF! What the &!*# should i do besides save up my money and run??