M.K.
These girls are also reacting to the lack of stability in their lives. They may need some counseling to help them understand what is going on with their mother. I hope their grandmother takes action to get legal guardianship of them otherwise mom can just waltz back in whenever she wants and stir things up all over again.
Children are very observant and react to their situations. If I were watching these girls I would have a strict schedule with lots of planned activities to keep them busy. Also make sure they understand what the rules are and enforce consequences when they break a rule. Children crave boundaries as it makes them feel safe. The more they know what to expect the easier it will be for them. It may get worse before its gets better if they are use to acting wild. But if you stay consistant you will see improvement. also talk to grandma about your frustrations and see if she can work with you.
Good Luck. These girls need a strong loving parental figure in their life and it looks like you might be it.