How to prevent son from picking up cousins bad behaviour

I live with my boyfriend and his two sisters and their bratty kids. Their kids are 3 and 4, and my son is 2. The one boy isnt too bad, the other boy is especially bratty. We usually just do our own thing away from them, but my son enjoys playing with his cousins and I dont want to keep him from them. I've noticed my son is picking up on his bratty boy cousins behaviour though. It's so annoying because he never used to be this way. Now hes learning how to hit, scream, and whine. The mom of the bratty boy laughed and said "it's nice to see that hes just like them, hahaha". Like, he literally picked up that behaviour from your bratty son. They also always force him to give them kisses and stuff which I hate. Moving out isnt in my near future unfortunately. I always stand my ground with my own son and tell him what behaviour is acceptable and what isnt, but then they always look at me with two heads like I'm a bad mom. I just dont want my son to be a brat after I actually take the time to parent him. Does anybody else have this problem? Any advice?