I can relate to how the OP and her family are/will be feeling, because 20 odd years ago, my grandmother and her partner moved from one end of the country to another, so that she could "help [my uncle] with childcare and housework". Said uncle was the baby of the family, horribly spoiled, in his 40s with 3 children - whose sole care he'd been responsible for, for some 10 years by this point.
My father, the oldest child, only found out that his mother and surrogate parent were moving, 2 weeks before it happened. And only then, because my grandmother's partner, fed up of the lies and so on, threw it into a Mothering Sunday conversation, in front of my parents, small daughter and myself, like a grenade. Like the OP's husband, my father was furious with the deceit, worried that my grandmother was reacting without thinking things through properly, and genuinely anxious that his youngest brother was going to use her for drudge work (which he did).
At the time, my grandmother would have been around the same age as the OP's FIL, perhaps a year or two older. She was in perfect health, as was her partner (who was 68, that I do know). They were both dead within 4 years of moving. And in that time, we saw them 3 times - and even then, my uncle and his children were always lurking.
When my grandmother died, my father was the executor of the estate (probably not the correct term for it) and he spent weeks up in the Midlands trying to sort everything out. My uncle's oldest son, then 16, and his girlfriend seemed to think that they were going to live (rent free, mind) in the bungalow which my grandmother and her partner had downsized into - and got nasty when my father and the middle brother told them "no". They'd made plans, blah, blah, blah... to which both my father and the middle brother told them "great, but unless you can afford the market price, tough!". The uncle whom my grandmother had thrown her entire life away for, so that she could do all the drudge work of raising his children (whose mother is, by the way, still alive and would have given her eye teeth to have helped care for them - she left their father, not them!) hired a skip and literally started to throw his dead mother and surrogate parent's belongings into it. It was shocking, and my father says he's glad I wasn't there, because I would have said/done something. I wish I had been there. I really do.
I was 22 when they moved. I had spent my whole life with them very firmly in it. We were close. I spent weekends with them, if I had a sick-day from school, it was my grandmother who cared for me, her partner and I shared a love of animals and walking, they took me on caravan holidays, to the seaside - and my grandmother even took me to theme parks, despite her fear of them in general. As a teenager, they vetted my boyfriends, attended my school events, treated me as a young adult rather than a child. They both adored my daughter, who was 3 when they left. She couldn't understand why we went from them dropping in to see us, or the other way around, to... nothing. There were phone calls, but in the pre-zoom days, they weren't the same as them seeing their great-grandchild growing up. And I will always have to carry the weight of knowing that the last time I ever spoke to my grandmother, she was sobbing - and she was a proud woman whose motto was "chin up!" - about how she was being treated by a man meant to be her son, and his brats of children. The last time that I actually saw her was 11 months before she died, at her partner's funeral/the day after it. And in hindsight, the way she clung to my father and me on those days? Makes me think we should have recognised the elder abuse going on. But we didn't. And my father was still furious about the lies she had told him to cover up her youngest son's plans to get the full market price of their several million pounds worth home (it didn't go for market value in the end). Me? I was just hurt that I'd lost my grandparents. In the end, she died of a broken heart. Her life-partner had died of cancer, she'd essentially lost her eldest son because of the way she played her hand, her youngest son treated her like a servant, and his children weren't much better.
I also think it scarred my daughter, despite how young she was. She won't have mention of them at all, 20 odd years later, because "it hurts too much". Perhaps she wondered if she'd done something to make them leave, in the way small children do. I don't know.
But, OP, if there's one thing I can say, one piece of advice, it'd be this: you cannot stop your FIL from behaving like an old fool - but you can counsel your husband and protect your children from the inevitable fallout. And there will be fallout, I'm afraid. Hopefully your FIL will go on for another 20 or 30 years - but we thought that about my grandparents. Life doesn't always go the way we expect it to. That's why sensible people don't make foolhardy decisions on the behest of grabby spoiled (wo)man-children!
