If they want a space that's theirs and theirs alone, they should be purchasing that - and not expecting you to be part of that arrangement. Are they going to share 'their' space with your sister and her kids?????? Because if so, then they are lying to themselves about the space being their space only. And the cleanliness argument falls apart when kids are involved - trust me on this.

If they only need a dedicated space occasionally, then this is what specialist medical AirBNB is for. Sort of Ronald McDonald House, but the adult version. Yes, this is available.