It’s called magical thinking.
The general advice is to not let the rules get more extreme. For instance, DD used to ask me, if I had cut the raw chicken and salad vegetables on the same chopping board? I was told to say to her:
”I am only going to say this once, and I am not saying it again. I chop the raw chicken, then I wash my hands, and do the salad on a different chopping board with a different knife. I don’t want salmonella any more than you do!”
PILs may not know what to do? I only knew, because DD1 was under Social Services for physical health reasons. (There are two aspects to Social Services - child protection, and the disabled). DH needs to discuss with PILs if they can discuss with SIL, if she would be willing to ask Adult Social Services for an assessment of need for herself.
PILs are legally fully entitled to say, they are not prepared to do any care of SIL any longer. There’s nothing Social Services likes more, than to put down the family will provide all care, which lets them off the hook. However, if the family refuse, or say they are only going to do x,y,z moving forwards, then Social Services has to assess her fully for social care.
As part of the assessment of need, SIL would have to undergo a financial assessment? If she were living in her own house, and needed carers to go into her house, she’s not entitled to that, if she has over £23,250 in assets, not including the house. There’s a sliding scale down to about £14,000 - if she has less assets than that, Social Services has to pay for all the social care. If as is likely, she’s not working and has no income/assets other than her benefits, then Social Services has to pay all the costs of her social care. She could ask as part of that, to be put on the housing list.
Then, regardless of what she does, PILs can ask Adult Social Services for a carers’ assessment on them. They are entitled to respite, and there should be a contingency plan for SIL, in case an emergency happens to them.
It is better for you and DH to get PILs to think about this now, rather than on the day of the death of the second parent. You would also be entitled on that day, to tell Adult Social Services, you are not going to do any caring of SIL, and she can’t live with you, so it’s up to them. Be firm. Social Services will do their best to blackmail you emotionally to look after her. No adult in England is legally obliged to care for another adult - the responsibility lies with Social Services.