1. You don't say whether he made a will, but I assume he did & the dispute is over carrying out its provisions. If so, your sis-in-law, as executrix needs to take legal advice. Ultimately, such disputes may have to go to Court to be settled, but this can be ruinously expensive & is to be avoided if at all possible.
2. I suspect that whoever told you there was a law permitting cases like this to ignore the terms of the will was talking through the back of their neck. Ask them to quote you the actual piece of legislation involved. In any case, your description of this woman (able bodied & middle aged) does not fit with someone who the State would have a duty to house - she could claim some benefits, but would have to look for a job. In that respect, she's no different from many thousands of other people.