Dog aggressive with special needs SIL. Help!

I have a 2 year old pitbull (let’s call him Zeus) that is SO hyper but sweet as can be. With our 4 year old, he’s always been incredibly tolerant and showed ZERO signs of being irritated with him. Anyways, we moved into my MIL’s house and my SIL is 18 and has autism as well as some other intellectual disabilities. She can be super aggressive and is often abusive to the dogs either on complete purpose or due to lack of impulse control. Kicks them, screams at them, drags them around, locks them outside, etc. She has a service animal (let’s call her Mila) that my MIL swears is super trained but is totally not 🙄 But she’s been around her from the jump which allowed Mila to have a high tolerance. Anyways, my SIL is incredibly abusive to Zeus and like any animal, Zeus is fed up with it. Any time she comes near him throwing a fit, he growls at her. Or if she starts screaming and throwing things at either myself, my husband, or my son, he growls and grissles up. Then my other SIL, let’s call her Kate, made things worse. A dog pooped in the house and she didn’t know what dog it was so she took it upon herself to try and rub every dogs face in it and scold them. Well Zeus doesn’t know her, nor have we ever done that with him, NOR was it his poop. So in turn, he panicked and scratched her arms really bad trying to get away. This has all painted such a bad picture and he’s such a sweet boy. We finally got into it tonight and my husband and I told my MIL that we will try to do whatever we can to calm his aggressive behaviors but she also needs to curb my SIL’s behaviors. My MIL stands on the side of “she’s special needs, people come first, she can’t help it, he just needs to be able to take it.” Our stance is, “He’s an animal, she’s abusing him, she CAN help a good majority of those behaviors, she also needs to be corrected”. Because she’s NEVER corrected. She curbs her angry feelings when it comes to humans so she has the ability. I’m also her staff member and educated on her disabilities and she does cognitively have the ability to stop those behaviors, at least for the most part. I just need some sort of advice. I can’t imagine getting rid of my baby 😭

SIDE NOTE: MIL is a vet tech and Kate is a groomer. So they claim to know everything there is to know about dog behavior.