I think my mother is complicit in child s*x abuse and let it happen to my son.

Hi all. I'm really struggling here. I'm a single mother of my 3 year old son (A) I've been living with my mother for 2 years since before my divorce. I have had sole custody of my son his entire life. He's been very well protected, cared for. He's stable and does well at daycare. Well behaved, intelligent, and kind. He's well adjusted to life without a father.

But my mom also has a boyfriend with 2 kids. A 5 year old boy (B) and a 4 year old girl (C). Apparently my mother, a RN, has had suspicions for some time about his kids suffering from abuse. The kids have preteen age brothers on their mom's side, but also B has a strangely attached relationship with the paternal grandma. She bribes the boy to sleep with her often and against the father's wishes. Until now they (grandma and father) have lived in the same house with 50/50 custody with the Mom.

5 days ago we moved into a bigger house so that boyfriend and kids can live there too. Kids would share a room, but closely observed due to her suspicions.

On Sunday, after a weekend of moving and packing, my little sister called me to tell me she's filing for divorce and I offered to come help since I'm half a year from my own divorce. She currently has like 10 dogs since their dog accidently got pregnant with an 11 puppy litter. They're 8 weeks and getting sold off but most of them are still there to litter the house with fecal matter and ammonia. I left my son with my mother and her boyfriend for the evening. No problems.

On Monday, boyfriends kids came to the house. I went to help my sister in the evening again, mom had offered to handle bedtime.

Tuesday is the shitshow. Kids got ready for bed. B ran around naked. A followed him into the master bedroom. Mom told me later that she felt a tinge of worry about that but DID NOTHING. I get my son (A) ready for bed. Book is ready. Lights are off. Adults say goodnight. I took a shower and got dressed in my room (all in the same hallway, master br is opposite side of the living room). Through the small opening in my door, which I did on purpose to check on children, I heard my son say "You're going to break my penis!"

Immediately I'm in there and kid B is running away from my son's bed. I extract my son to my room and have a talk. Im not angry, I'm not shaming, I'm very level headed in stressful moments and handled this like any other teaching moment. This was our convo:

Me "why is your penis out?" A "I dunno" Me "who's idea was that?" A "I dunno. B" Me "did B touch your penis?" A "No, he just licked it." Me "oh nooo those are private parts blah blah teaching explanation about privates and who's allowed and only bathtime nudity and privacy etc very calm, not freaking out. Sleep in my bed tonight."

I got across the way to tell my mother what just happened. She sighs, does not move and says "I knew this was going to happen. [Boyfriend], your son just licked A's penis. I think your children are abused."

Like what kind of reaction is that???

Boyfriend texts his baby momma accusing her of being at fault for B's learning of such behaviors, yells at B, and extracts girl C from the room for her to sleep on the couch. Baby momma is now firing back that it's My son's fault. Moms Boyfriend gives no apology to me on behalf of my kid. Has not said a word to me since.

I call my best friend and we talk for a while to figure out what happened, how and why they reacted that way, and talk through my own confused and angry feelings. On the phone I remember that I was helping my sister on Monday night. This could've happened on Monday night too while my mother and her BF have their door shut, not watching the kids.

Next morning, I'm ignoring my mother. I don't want to talk to her until I'm talking to both of them about how to handle this. She refuses my ignoring and gotes me into an argument. My son is at the kitchen table while I fix him breakfast. Here's the gyst of that convo

Me: What happened Monday night?

Mother: nothing happened! We put the kids down! We went in there half a dozen times to be quiet and stop moving around and...nothing happened that I know of.

Me: that's all I need to know. Right there.

I asked why she didn't bring the children that need observing into their big bedroom and let my son sleep in his own bed. She raises her voice to say "BECAUSE I BOUGHT THIS HOUSE FOR THEM. THE PROBLEM IS YOURE STILL HERE." (This is part of a larger problem within our dynamic. She invited me in due to the financial hardship of solo parenting in this economy, then hated me for it) I replied that she knows nothing about this man or his children or his family or his past and she takes more care of strangers than her own family. She's choosing a man over her own.

At this point she gets in my face, like all up in my face, to call me an ugly bitch and I should shut my ugly bitch mouth. I brought my arms up to block my face since she was posed like she was going to hit me. She grabs my wrists and I tighten up. She's yelling at me with verbal abuse type of words, trying to make my hands hit my own face, rearing back with fists like she's going to punch, yelling the whole time. She slams me into the refrigerator. I'm choosing to simply hold my stance and not react. I wanted to hit her but... Remember, my 3 year old son is sitting at the table watching his grandmother try to beat up his mother. I point out this fact to her and tell her that she will be known in the brand new neighborhood as the New Crazy Lady with No Family. As in, I will go away and take my son and my siblings (21 &19) with me. She storms off screaming at me to shut my goddamn ugly bitch mouth.

I went to the police and filed a report. The Boyfriend has apparently gone to the police also to file a case with dfacs for the boy.

I cleaned my sister's house more and when I told her this happened, she said without hesitation for me and A to move into her spare room. I'll help her with her bills and get the house pretty and ready to sell. She will help me with childcare so that I can get a second job or take more shifts to make more money. Something my mother would never do because shes "not that kind of grandma".

My mother is now blowing up my sister's phone asking if she's "taking me in" and warning her that I'm a "parasite" and I incited her to anger and what happened to my son was my fault.

I'm sorry that this is so long. I'm at a loss for how to feel or what to do. Am I over reacting? Am I under-reacting?

Tl;dr: My mother's boyfriend's son molested my son, she's blaming me for it happening, got violent with me, kicked me out, what the fuck do I do with that?