I've posted about my PILs doing this.
DH grew being told that his mother would have a mental breakdown if he (or his siblings) ever upset her and that it would be his (or their) fault.
The entire family would have you believe that MIL is depressed and delicate, dedicated to her family and just a bit thoughtless and silly, doesn't think before she speaks.
What she actually is is selfish, manipulative, angry, passive-aggressive, jealous, controlling and downright bloody spiteful. She likes to be the centre of attention and if she feels her limelight is being stolen or someone is not giving her the attention she deserves then all hell breaks loose. She cries and weeps and makes everyone feel guilty while FIL rings up to scold the offender like a naughty child or shout them down if they dare to argue their point.
They have been like this for years and I fitted right in OP, I took my place and was afraid to upset her. I married DH very quickly and I didn't want to rock the boat with the rest of the family. I let her insult, upset and manipulate me for years before I finally spoke out.
Now I have no contact with PILS and it's a big relief to me. But the rest of the family are not happy about it and even though she did and said some of the worst things possible, I'm the one who gets the blame.
I was reading a book called Toxic Parents (and have the follow-up, Toxic In-Laws being delivered soon) and it made a lot of sense. The section that applies to us is the one on Controlling Parents and some of the families discussed in the book sound terrible.
One poor man won a a Christmas holiday for him and his girlfriend and his mother went into meltdown. But while she was the one who wept quietly and said she would be okay, even though Christmas was ruined and she wasn't going to cook a dinner, it was his siblings who got angry and told him that he had ruined what could be her last Christmas. I think she was 56 and in good health, so not on her last legs or anything.
The book describes how a controller wants to have things their own way but also thrives on the chaos that takes place if anyone ever confronts or stands up to them.
While people are afraid to stand up to them, they are ruling the roost and getting their own way. When they are not getting what they want, or someone challenges them, they have the opportunity to get upset and cause a fuss (often by crying and saying "I don't want to make a fuss") and everyone else flaps around them like headless chickens, trying to make them feel better and putting the other person in their place.
It says partners and siblings will often gang up on a child or sibling who has upset the controller, partly for a quiet life and partly because they don't want to question their own behaviour and look at why they are happy to let the controller treat them badly.
A controller will say something nasty just to test the waters, to see if anyone will challenge them or not. If nobody does, they are happy knowing they are still on top, if someone does say something they can enjoy being the centre of a drama no matter what the resolution may be.
And I think that's right. My MIL hasn't gotten exactly what she wanted with me, she has been challenged on her behaviour and she doesn't see me or our DS anymore. But she does get to talk about me with the rest of the family and be comforted by them because I am being so awful and won't accept her apology and didn't use Christmas as a turning point. Which is still, according to the book, a satisfactory outcome to the chaos she has caused. She can be upset and I can get the blame, even though her own actions were the reason for the conflict, and she can use their feeling sorry for her to manipulate them.