absent other grandmother

Something is going on with other grandmother. On mother's day weekend I am told she chose to be away from her home when grandbaby's mama had the kids at her house. Mine is supposed to be there with supervised visitation. Other grandfather was there, in and out. So I guess that counts. Now this weekend, first the mama was just coming on Sunday and was just coming to our house in preparation for the girl's pre-k graduation. Last night she calls to say she is coming in at noon today and wants to pick the girl up. I say ok. She talks to the girl all about it, makes her happy, gets her excited, then tells me other grandmother won't be there again and other grandfather will be out of town with a ball tournament. I told her we would talk about it later.
That I didn't understand why her mother would do that when she knows it's supposed to be supervised visitation only. If I say no, I look like the bad guy to the girl, not letting her stay with her mother since her mother already told her she was coming,etc. That is not right, I feel like they are playing me. If I say yes, then I am going against what the lawyer and the psychologist have said. This is the second time it has happened. I think I will tell her it can't happen again or there will be no more visits until the issue can be straightened out. At the same time, I need them to help out with child care during the girl's dad's surgery and hospital time that is fast approaching. I wonder what other grandmother thinks she is doing? getting a well deserved break from extra duties or making her daughter take care of her own kids for a change, or what. WHAT.