I posted this months ago and removed it quickly after suddenly fearing my MIL would see it... but who cares I guess! So here we go.

I will start by saying that I am sorry for any grammatical errors, English is my second language, after French. Heads up, this is a long one but I'd appreciate the insight.

I (30yo female) and my common-law partner (34 year old male) had our 14 months old baby looked after for the first time by my mother-in-law (54 yo female) and my sister-in-law (35 yo female). ***We will call them MIL and SIL for simplicity (I got backlash last time, since we are not married yet). **

I was very insecure about leaving our baby with MIL, but she was our only option and we needed a sitter that evening (my family lives on the other side of the country and we are both shift workers who couldn't reorganize our schedules this time (HCP and Law enforcement).

My son doesn't see his grandmother much, although she lives 9 min away. She doesn't come over to spend time with him. When she is at our house and her other grandchildren (female 2.5 yo and female 6m) are also present, she will take care of her two granddaughters but is mostly uninterested in our son. I assumed that this was related to the fact that her daughters require more help from their mother (both are no longer with the baby-daddy, one of them works 70+h a week and the other is on welfare). To be fair, I'm ok with her not interacting as much with our son because some of the family patterns between MIL and my two SIL seem a little toxic. Nevertheless, I make an effort to involve my MIL in our life. I send crafty cards for holidays, send pictures, message her about important milestones and invite her over to bond with my son.

The first few years of my relationship with my partner, my MIL did not like me, out of solidarity for my partner's ex-girlfriend whom she adored... At the time I respected her grief of the relationship and thought she would eventually come around and she did! She apologized for treating me poorly.
(Context: My partner and I have been together for almost 5 years. He and his ex were together for 1 year and they lived in different states for 6 months of that year. I am not the reason they separated. )

In the past, my MIL broke my trust. For example, I saw her give a bottle of formula to my niece while she was helping her daughter in the postpartum period. Her daughter had specifically asked to be woken up to breastfeed (I minded my own business at the time, not wanting to cause a scene).
A few years later, I witnessed my MIL giving meat to my vegetarian niece, behind her own daughter's back. At that point, I felt like I had to disclose this information to the child's mother out of respect. This situation got out of control. My MIL was livid and lied about the incident, stating it never happened. We talked it out and I took most of the blame for ''not minding my own business''. Things were ok after that or I thought...

But since that incident, she has not responded to any of my text messages but will send a response to my SO instead.
She organizes all family events by checking in only with SO, knowing full well that I am the one who manages our family calendar and she used to come to me.
At family dinners and events, she acts as if she likes me very much.

Going back to the babysitting story...Before leaving, we gave only one instruction: do not put our son to bed. We had specified several times that we would put him to bed when we returned and made sure he napped later that day to accommodate for the 1h later bedtime. We explained that our son was going through a lot of changes with my recent return to work and that we preferred to keep his bedtime routine with dad, to reduce his separation anxiety. She accepted.
(Context: I took 12 months of maternity leave and my SO works mostly evenings. I breastfed and took care of all sleep duties. Our son was having a hard time adjusting to me being at work, as well as teething and showing the typical ''fear of stranger'' at this age)

My partner and I had security cameras installed in the house after a break and enter (MIL did not know). We did not intend on eavesdropping during the evening, the cameras are always on when we leave.

A few hours later I received a message from my MIL: she put my son to bed, stating that he had not cried at all and fell asleep in 8 minutes (context: that's how long it takes me, she knows this). I was frustrated at this point and felt that my boundary had not been respected. I also had doubts that it went as well as she said. I was ashamed but when I got home I re-listened to the bedtime routine to see how it went.

Shocker! She lied to me, he cried so much, he screamed for his mom and it took her forever to put him to sleep. He fell asleep from exhaustion. My heart broke.

With my doubts confirmed, I listened to the rest of the evening. Not only did she purposely put him to bed ONLY to make me angry, we can hear MIL say to SIL : “Alright! I'm going to put him to bed. She’s going to be livid! '' she then proceeds to laugh.

Once our son is asleep, she proudly tells SIL that she’s going to text me and she is impatiently waiting for my response, then she laughs and calls me batshit crazy.
(context: I responded something like: he was very tired today, that must have helped him settle. I'm glad it went well. )

She spent the evening belittling me to my SIL.
She tried to get my son to walk so that I would miss his first steps “because it would hurt me” and then started laughing.
My son was crying and looking for me and she never reassured him. She tried to make him say “graaaandma” everytime he cried out for his '' mama''.
I shared this recording with my partner and he was understandably angry and very hurt.

He supports me all the way. He confronted his mother and chose to no longer have a relationship with her.

She did not deny any of it and said she wouldn’t apologize since she meant all of it and that would make her a hypocrite... She also said I was trying to take her son away (he wants to move 40 min away to the country).

I offered to hear her out and talk... I won't forgive or forget, but I'd be willing to be civil if I needed to be in a room with her. She will NEVER babysit or be alone with our son ever again.

It's been 5 months and we haven't heard anything from her. What should I do?

I feel like the longer we ignore this, the more she's building hate towards me, the more my SO has a broken family and she's also lying to everyone about why she hasn't seen us in months.