Is this scenario really that odd? The mother has moved elsewhere but still has this property on her hands. Letting it seems pretty reasonable in the circumstances, and given what a leap in the dark taking on a tenant can be, what better solution than to let to a family member you know well and who needs a place to live?
Emma’s suggestion of an ‘intervention’ to check on payment of rent at some point in the future seems reasonable. I really do not see what else you could ask for at this stage.
