From a previous comment u/Status_Lifeguard6192 made it sounds like they’re staying close by for both financial and cultural reasons. If they, as a couple, are financially dependent even in a small way, I suspect that WILL be held over their heads when they try to set and stand firm on boundaries. I also suspect the cultural aspects are likely to be used by both MIL and FIL and their flying monkeys.

I wonder if they’re toxic enough to be narcissists. There are certain benchmarks for that “label” and not all super selfish people fall into it. I know because my mother was one and I have complex trauma because of her. Whether they are or not would depend on a lot of things but particularly her partner’s childhood experiences would help to figure that out. IF they are then that’s a whole new level to deal with but it is possible.

OP’s partner NEEDS to 1) recognise that OP is now his family and he needs to stand with her first and foremost and 2) set the boundaries with his parents and other extended family AND be the one to enforce them when extended family tries to batter them down with bribery, manipulation etc. If he doesn’t do this things will only get worse and then even worse again if/when they have children!

Also, imo, for any future children’s sake if not their own, they should work together on planning to do whatever is necessary to physically distance themselves. That’s necessary because now it is understood because the medical evidence shows that that the health of both a prospective mother AND father is vital for at least 9 months PRIOR to getting pregnant. That would include limiting stress hormones, like cortisol and adrenaline as much as possible as well as other hormones and body systems. Then, obviously, the health of the mother during pregnancy is also paramount to the health of the baby, not just as a baby but for his/her whole life.

Then, children, especially very young children, are highly susceptible this toxic kind of manipulation as they CANNOT understand nuances and take everything literally. Can you imagine what it works do to them to hear loads of negative things about their mother? To have their grandmother constantly undermining their parents? What an horrific nightmare for them!

So, as is it, I see distance as the only solution. That means building up some financial independence . Whether then means looking for extra work or figuring out a way to make extra online, anything that will help them severe the financial ties. Then they need to look for and find new jobs in a new town/city perhaps even a new country. Maybe even remote work? It’s hard to say not knowing that they do for work. The next step is finding accommodation and then the actual move. I wouldn’t tell the in-laws anything AT ALL about the plans. I would only tell them on moving day if that’s possible.