If you are able to allow them to live there rent free, you are sure it won't cause trouble with your other child and it gives you pleasure to do it - then do it.

You could charge them a nominal rent, then put it aside for them to use later. You could use that small amount for home repairs, property taxes, etc. Or let them live there free, but ask them to pay towards any home expenses (repairs, maintenance.)

But, it's your house, you can do what you want!

It sounds like you are a very caring mother, good for you!
Kelly