I agree with the majority.
I think that by making that promise, you've done the equivalent of signing a contract with no quid pro quo - a promise with nothing in return. As I understand the law, such agreements are unenforceable. I don't know if you consider that relevant here or not.
Do you feel that you can you tell her that you changed your mind, perhaps at the urging of the spirits, or do you feel that that's no longer an option for you?
Here's my suggestion: If you feel like you must let her pray during your seance, ask her to do it from her home. If you have promised to let her be there, have her go into another room, close the door, and pray silently. If you feel like you have committed to allowing her to pray visibly during your seance, you're stuck between giving her what she wants or welching on your promise. Under no circumstances should she be allowed to disrupt the proceedings, and her very presence visibly praying there does that.