Quoting Tiger Lily ♥:" I'm sorry! I think you need to put your foot down. Tell your mother, calmly, that these are YOUR children and that you expect for rules to be followed."
Let her know that she needs to respect your position and rules.
Ask her, how would she would feel and what she would have done if another person went against her rules and wishes with her kids?
I don't know a single mother who would put up with that...even from her own mother. If my family went against my wishes, my children would not be visiting until they learn to respect my wishes and rules.