Quick question about taking over a mortgage from a family member

[Henry Uili]

Quick question 

So here’s the scenario. A family member has her daughter and son in law living with her. They want to take over the full mortgage payments to help free the parents from debt and also keep the parents as tenants to come and go as they wish. They want to structure an agreement where the daughter is added to the mortgage to make it official and That’s all I got for now. 

So how can they structure a deal or agreement with that information.