I suppose with your recent posts things definitely shift somewhat.
An awful controlling mother and they were closer to their father.
The marriage wasn't great in the first
place and your FIL had little interest in caring for her.
A very sad ugly situation no doubt.
I still feel great sympathy for your MIL as it is an awful situation, but it is exacerbated by her children having understandable resentment towards her.
If only parents realised the damage they do long term to their children when they behave like this.
This is a topic in our house as one of my daughters parents are like this and at 17 she is clashing so much.
As she is a really quiet child, afraid of boys, they are shoring up huge problems.
They are nice people, well the mother is, not so sure about the father, but my daughter tells me she can't wait to leave their beautiful home to just get away from their controlling ways.
No phone allowed EVER in her bedroom, which seems a bit OTT, is one of many rules.
Her friends can't fathom this of course.
OP, you may need to pull back from your MIL and put some space between you.
Her controlling ways may well have morphed into fury and it is not good for you to be around it.
I think you need to put your children and yourself first.
Making her 5 year old grandchild cry on his birthday is definitely line in the sand that would be the start of new boundaries going forward.
It is best to be clear on this as it woud be such a shame for her to lose even more than she has.
However painful this is for her, she needs this harsh truth to be told to her before it is too late.
I would be very upset at my child being collateral damage in this.
Completely unacceptable.