What do I do?

My SIL is 26 and has a 5 year old boy, she's constantly leaving him home alone for hours at a time so she can drive to bring people weed/ doesn't set up a babysitter for after school. He poops his pants often lately, no one is sure why because he hasn't had an accident in a couple years. Her apartment is a mess and she's constantly drinking around him and having parties, she got a dog two months ago and I go over to pick up my nephew and there's always dog poop by his toys and all up the walls by the dogs kennel. She makes him clean it up whenever he mentions it. She's been investigated multiple times by cps. I don't want him to be there, he's a very sweet boy and she's always yelling at him. Me and my husband have tried talking to her calmly and she freaks out thinking we're calling her a bad parent. What should we do??

Added on: we are willing to take him in we just aren't sure how to go about it.