Bad influence on kid

So my nephew is 3.5 and he goes to the same sitter as my son who’s almost 2 now. Just an in home sitter. They go mainly on opposite days so they might see each other 2 times a week max. There are 2 days where my SIL picks him up and she keeps him about 45 min.

My nephew is a little shit (to put it nicely) and he rarely gets disciplined. He is a lot to handle. Even the sitter says he has a bad attitude and doesn’t listen well. My concern is that I’ve noticed my son throwing little tantrums and being a brat on days when my nephew is there. He is normally super sweet and lovey. He rarely ever gets upset when told no until he’s been with my nephew.

I really feel like my SIL is fueling this behavior because she lets her son act this way and mine just follows along to whatever his big cousin says. Just today at their house, my nephew was standing on the coffee table and jumping onto the couch or floor and back on the table. They didn’t say a word to him at all. I of course, wouldn’t allow my son to join in. I’m just honestly scared of what spending time with my nephew will do to my son. I love my sitter and she does what she can, but she can’t control my nephew either. How do I even go about this situation? I mean basically someone needs to tell my SIL that her parenting sucks and she needs to discipline her child, but that’s not exactly the best plan. I just don’t know what to do. My husband and I are very concerned