I need advice

Okay, some of you may remember me posting something like this a few weeks back. I posted about how I live in a house with my boyfriend, our two year old son, and my boyfriends sisters two bratty kids, both 3. So anyway, I pretty much dont know how to go about their bratty kids regarding my own son. My son loves to play with them, but they're both so aggressive and mean to him. When he happens to take a toy off the floor and one of them wants it, they'll snatch it so hard from him he'll jolt forward. The other one says "you cant play in here, it's for me and my cousin only", etc. I'm just afraid they're going to turn my son into a brat. I also dont want him to be mistreated.

My son is allergic to peanuts and one of the kids yesterday (the brattiest one) asked if my son can eat what he was eating. I said no, I dont know if it has peanuts or not. He then took my son and closed the bathroom door quickly and tried to give him one. I opened that door so quick I didnt care if it hit him in the head tbh. Our room is in the basement and they'll constantly open and slam the door and laugh until their parents come and yell at them. Btw when all of this is happening their moms are outside having cigarettes for 20 minutes+ at a time. Everything in me keeps me from slapping them into outer space.

If they do something bad and their parents happen to see it, they just say their name in a stern tone and go about their day. Sometimes they threaten them but never follow through.

I guess what I'm asking is, should I just keep my son from them? He always asks to play with them so I feel bad and let him go play (with me watching). Sometimes they play fine (only when their moms are in there). But I honestly hate how they go about discipline. When one of them is doing something bad I tell my son not to copy that behaviour because it's bad (outloud so the moms hear). Unfortately moving isnt in my near future at the moment.