We have a vacation coming up that we planned quite a while ago. We rented a house on the ocean for a week with my in laws. Last week DH's cousin invited herself and her 3 year old son to join us. I like the cousin, she is nice, and generous. The problem is her son is a bit of a bully, he plays very rough, whines constantly, and his parents don't do much about his behavior at all. Honestly, they don't even seem to pay much attention to what is going on. DS is 1 and when we've seen these cousins before he's very nearly been hurt by this boy - grabbed tightly around the neck, nearly smashed in the head with a toy... you get the idea.

I have very little experience dealing with other parents in situations like these. Any advice for how to handle the mom or the boy when issues inevitably arise, or before they do? I don't want to create unnecessary conflict within DH's family, but I also don't want my kid to get hurt. Is there a nice way to say "get your kid under control" or "keep him away from my kid if he can't be gentle"?

Sorry for the long post - I want to not feel stressed out about my vacation, so I'd like to have some ideas in my back pocket.