I know of someone who did this. He moved his family into mom's house, telling his siblings it was to 'help' her. He then had mom moved out within a year.
Then, mom passed without having her affairs in order, and the family had a huge fight during probate because the son wouldn't pay for or give up the house, saying he had earned it. He eventually paid everyone off, but at far below market value.
I'm not saying this is what's happening with your sister-in-law, but I would definitely find it concerning.