I am not a parent so if what I say sounds harsh or wrong, IGNORE IT.
Here is what I would do:
1.) Set your boundaries for YOU and at a level you can tolerate. You are Grandma, not Babysitter. You have YOUR OWN LIFE.
2.) Let her know what the boundaries are.
3.) Let her know you will call CPS if she does not get running water within X days (housing is not decent, safe and sanitary).
If she can afford a picnic, she can afford a sitter and if she can afford a picnic she can afford to get running H2O. If she does not have enough money there are people who work part time for extra money. I know. I worked 3 jobs for 4 years.
If I am off base it is OK to tell me so. I may be out of line even saying any of this in the first place!