The sale of the house can have huge implications in the future, in particular for the mum. What happens if the couple who owns the house decides to separate, faces financial difficulties, sickness, needs to change location for a new job, one of them becomes a widow ... what is then the situation of the mum. What happens if the arrangement doesn't work out (conflict of personality or anything else)? Would the mum need then to find alternative accommodation? I know that these could or could not happen. But very few people don't face any of these difficulties in real life.