If this was done to your child what would you do?

So I don't want to make this too long I'll try to keep it as direct to the point as possible without too many details. So my husband and 2 toddlers live with my grandmother but she has her house and in her backyard there's another small house (which we live in). We pay rent btw. Anyways, she let her son who's homeless "stay for the winter" since November. Ever since he's moved in it's been nothing but problems. He always complains that my son cries alot (mind you, he just turned 3 ). Every little thing my son does is always wrong to him. He complained to my mom that he couldn't "count past 5" (he can count up to 25 FYI and he speaks in clear full sentences ever since he was 2 he's just shy). Well anyways he's gotten up in his face mocking him and making faces because he's crying. He also complains about my kids fighting all the time . They're literally one year apart and siblings fight, it's normal. Well today my kids were outside fighting over who gets to be in this ride on car and I had separated my kids already , I walk away and hear them fighting again so I walk out again and walk my daughter to our house to stop the fighting. Not even 2 minutes later , my mom comes to my house telling me that my uncle threw my son's car on the roof . Minuets later there was a big argument between them. I've always been a shy person and never stand up for myself I think it's also because of my social anxiety but this really pissed me off. Physically throwing my son's car / screaming at him was a big NO to me, that's crossing the line. I feel like he needs to mind his business and accept the fact that kids fight and he's going to have to deal with it . These are my kids not his. His kids don't even talk to him because how physically abusive he was to them . I don't understand why he picks on my son all the time but when his precious granddaughter comes over and throws fits worse than my son he won't say SHIT to her. 🤔 This uncle of mine is around 50 years old btw . And he complains to my mom how she's not the kids "babysitter" yet, I clean both houses, cook, do mom duties, and work while his dumbass doesn't do shit but smoke weed all day. I really don't understand why he's worried so much about MY kids.