I need to keep my almost 4 YO nephew away from our daughter; but I don't want to have them hate me either?long?

My sister in law has a almost 4 YO and is due tomorrow with her second. My husband and I have a 2 Yo daughter. My daughter is generally very good at playing with others even though she doesn't have anyone to constantly be "sharing" with ect. She LOVES her cousin and loves to see him/play with him. He HATES her. He is a monster all the time; but he especially has a problem playing with girls in general. At his daycare he is always mean to the girls more then they boys ect. And as it is he rarely plays with others even at school. My SIL is a terrible mother. She desperatly wanted a girl with him and with the baby that is about to be born, but both are boys. She certainly hasn't been a 'good" mother. She lets the TV watch her son while she watches TV in the other room. THis would be 100% different had he been a girl. I try to keep my daughter away from my SIL and her cousin as much as I can because they are not good people. They are total low lifes who take advantage of my enabling MIL. My MIL is another story. She and I have never gotten along. We are generally civil and have occionally had a few good times but basically we tolerate each other. She thinks her grandson is the most amazing thin on earth, as she should; but well beyond the tolearble point. My MIL and FIL have my nephew ALOT like 50% of the time and when he was younger probably 70% of the time. Granted my SIL is also there with him most of the time; but not always. Being as they take care of him so much and see how unmotherly my SIL is you'd think they'd make an attempt to instill some basic thins in him. Like sharing and not hitting ect. And just being a decent person in general. But my MIL says it's not her job. TO me when you have him as much/more then his parents, it sort of becomes partly your job. He physically harms my daughter in front of my MIL ans she shakes it off as 'o he's only 4 he doesn't know better". Well at 2 my daughter knows better. She dropped a ball that literally touched his leg and he turned around standing less then 2 feet away from her and chucked a basketball at the side of her head. Thank god she was OK and my husband and I were there to take her home immediatly; but we had been inside while my MIL was watching them. All she said to my nephew was that it wasn't nice. WHen I made a bi deal out of it she said "well she hit him with a ball". I responded "no she dropped a ball that touched his leg that is NOT the same" and she literally shrugged. My daughter is never alone at my SIL's house. But occionally my In-laws watch her (never for longer then 2-3 hours). However I never know when her cousin will be dropped off at the same time. I don't want to keep her from her grandparents because she loves them and they love her. However my MIL seriously doesn't do anything to protect my dauhter from her cousin and tells me "they're kids it's fine". Well they may be little and yes accidents happen but to blatently disregard her safety is not OK; just because he was their first grandson and he is so close to them. I don't know what to do. With her baby about to be born I know I will be asked to baby sit the 4 YO and I honestly do not want him here. He is destructive mean and does NOT listen to a sinle word you say to him. How can I put my foot down. My husband agrees with me and will speak up if need be; but I need to "prompt" him in what to say sort of. And alot of times he backs down from my MIL. So I need to know how to take care of this myself in a way to make her not hate me.