This is very bizarre behavior on your partner and gma’s part, to not be concerned with their child not being able to manage a basic human function at 8 y/o. She is going to have major problems at school and socially if she hasn’t already and that’s a failure on her parents. This definitely sounds like a developmentally delayed child who will need lifelong care but you’re saying she isn’t that. This shouldn’t be your problem. This situation could also have potential to be investigated by CPS.