1) It sounds like the place has let her live there for 5 years or more before making an issue of it? Isn’t there some legal principle (estoppel or something) that keeps them from selective enforcement?
2) It is possible that the grandparents never anticipated she would be with them “permanently”, thought the mother would rehabilitate and take her back, so they did not rush to sell when she was first placed.
Even if they were hoping mother would get rehabilitated, a year of waiting would have been sufficient.
The market was still doing well 4 years ago.
stop injecting rational arguments into this discussion! these people will have none of that!