Rent to own a family members house

[AJ Howard]

My moms credit score is not the hottest on the block but it is improving. She wants to buy my gmas house (her moms). My grandmother wants her to pay the mortgage, so in essence, rent from my grandmother until their credit rises and they can have the score they need for the loan. But they don’t want her to turn and sell it during a mood change. How can my mom and grandmother both be protected? Anyone have rent to own contracts? Like to buy within a year? Etc.