See, in my experience with controlling people, what you’re laying out leads to either one of two things:
(1) Exhausting as OP has to be totally subservient to the MIL all the time, because she has no boundaries and everything is ok.
(2) More and worse clashes in the future.
That’s the fundamental problem here. She simply can’t live with the MIL behaving like this. No one can live in a healthy way with someone else behaving like this. She will have to set a boundary sooner or later, unless she intents to turn over not only her husband but her children to this woman. If she doesn’t do that early, it’s going to be a worse battle. If she is half-hearted and allows some inappropriate behavior, what that teaches is that the behavior is ok sometimes - so the MIL will keep pushing even harder when they do say no, because sometimes it’s ok, and maybe if she pushes hard enough OP will give in!
Boundaries aren’t some special thing for difficult people; they exist in any healthy relationship. They’re often unstated or only lightly stated, because in a healthy relationship each party respects the other’s wishes. But sometimes when you get someone who doesn’t really see the need to respect others. That’s when they become more explicit. But they’re not an attempt to get angry or control the other person - simply to define the relationship in as healthy a manner as you can.
You’re still trying to treat this like a normal, rational person. You can’t do that with people like this. It’s like dealing with dog, in many ways. If you don’t want the dog to beg at the table, you must never ever feed it scraps from the table. If you feed it scraps sometimes, but say no other times, you will have to deal with more begging the other times.
Believe me, I’ve lived this with a controlling parent. That’s why I say these things. Because I tried the sort of approach you suggest for years, and that’s exactly what happened. I found myself meeting her halfway so often that we would end up meeting right where she wanted to be all the time. It got to the point where I was literally physically shaking trying to deal with her, while at the same time being yelled at for how awful and selfish I was being for going against her and how if I’d just let her fix everything my stress would go away. To where I was repeatedly saying no and the only thing that no meant was “yell at me more until I say yes.” (I would also say, with things as they are, there’s a high chance of MIL ending up living with them.)
I would want to bring the husband on board if possible, yes. But I worry that this could turn into a situation where both OP and her children are in an untenable situation. And frankly her husband shows signs of not entirely thinking clearly. Parents like that, they raise you to think that if you’re not a subservient child to them all the time you are a horrible hateful person who probably just wants their parents to die so they won’t bother them anymore. It can actually help to see that the world doesn’t end when someone else says no.
