I think the moms mom has enabled the behavior so long the child's mom and dad expect the support to continue.
I would advise the moms mom to stop supporting them. To throw down some ground rules and give babies mom the choice of abiding or moving out. And like, immediately. If she chooses to move out give her a day or so and then sell the items she leaves behind to recoup some of the $$ lost on supporting the daughters lifestyle for so long.
Then I would say no when daughter asked to move back. If child is in danger then call the authorities.
Or it will be a cycle.

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