My wife slept with her sisters boyfriend

My wife spent the night at her sisters on saturday the 18th and took our 7 month old son so I could get some sleep because I work weekends. Her sister and boyfriend rent a single bedroom out of a guys house about 40 minutes away.

I come home Sunday and at they are not home. She says she wants to spend one more night which I agreed to. Come monday morning she admits to me that she slept with her sisters boyfriend on Saturday. Her sister wont kick out my wife because she has our son snd that's her sister. She won't kick out her boyfriend because she is too submissive/afraid. My wife has been drunk most of the time since then. I was supposed to pick her up after I got off work, but I was in such emotional shock that she knew it wouldn't be a good idea. While I was at work they came with a police escort to my mothers to get some of the babies stuff. Tuesday and today we mulled over the ideas of marriage counseling or divorce.

She allowed me to bring my son home to my mothers house where we all live. I am afraid come friday she will come and take my son back to where they live with her sister and boyfriend. They drink, smoke marijuana outside afaik, and smoke nicotine inside. Is there anything I can do to keep my son with me if she wants him back? The living situation over there makes me fear for my son's safety. This is in illinois where weed is legal.

Apologies if this is in the wrong sub.