I think many of you are missing the point. the house has only 2 bedrooms, where would the OPs grandchild sleep? A 2 bed house probably has only one living space and the OP would have no extra bed/ bedding and what about extra costs for washing and baths/showers etc.
She might apply for child benefit to be awarded to her instead of the mother but that wouldn't offset costs. Does the OP have a husband at home? That would make for a crowded house.
Would she out her son out? He's not vulnerable so would be way down the housing list. Is father working? He could rent privately but would daughter go with him? Maybe he likes pleasing himself more than having to look after his daughter.
I feel heartily sorry for this child, it must be terrible to feel that no-one wants you especially a parent. I wish I knew them as I'd give her a room myself. It's heartbreaking.
Passports not in the drawer I always keep them in. Turning the place upside down.