Apologies to sound harsh and uncaring, but if anything I am significantly understating the severity of the situation and at this point my primary concern has to be protecting DC.
We have this situation with SIL. Diagnosed MH issues, but has never got help or left home. She does work in a limited way (via a Gov route - not UK - that makes it practically impossible to curtail, despite conflict), but struggles with most people and is also extremely difficult to be around. If she doesn’t like a situation (not necessarily a predictable reason why) she will manipulate it at any cost. Rages out of nowhere. DC are terrified. I have drawn a boundary to protect DC. DH is pathetic and has not been able to find and hold reasonable boundaries, that would actually enable us to provide support. I would divorce him but don’t want shared custody with the potential for unsupervised exposure to the aunt a big factor (I have a lot of sympathy, but the rages and manipulation can only be described as terrifying - apologies to sound so harsh, but it is the reality).
PIL seem trapped by it. SIL basically controls any access to GC, so PIL don’t really have a relationship despite living nearby - we absolutely do not want childcare help, as DC could not be exposed to this). SIL also blocked home help from the house when PIL needed it. I did intervene/get DH to intervene at this point.
What were PiL thinking about the future. How could this ever end well? They are now reaching the end. There isn’t really a plan for what will happen. A decade or so ago I thought they were kind and protective. Now they are reaching the end it is clear the situation is dire. I worry they may just expect her to move in with us, but it would be decimation of family life and a legitimate concern (based on things that have happened) would be danger to DC.
It horrifies me to think this is not an isolated case. I guess provision of care is majorly lacking for people with significant MH issues who do have some independence. In this case when the family protects (shields) them and do not encourage independence it is at a huge detriment to the future of the person kept at home. Maybe this level of shielding is because of decisions made decades ago when concerns/stigma was different. Siblings and their partners will often have to step in when they have children at home.
YANBU. Independence has to be encouraged and plans made for a secure future. People cannot be shielded until parents reach the end, as the situation is likely to get worse.
YABU. PIL did the right thing.