Make your feelings on the matter clear, as though you had written them in indelible ink on the living room wall. Regardless of the act, the principle is that you told her something and she both purposely ruined the experience for you both and went against your clear instructions.
Yes, today is the banana. Tomorrow, another boundary and another first could be lost. Stand up to her, you are the only one who can advocate for your daughter.
Sit down with her and explain why you are upset and what feeding her meant to you both and ask why she fed her when you expressly said a) no bananas and b) you were to begin feeding her tonight. Do not use your child as a weapon by withholding contact. That's not right.
Do not brush this under the carpet and do not put off confronting her regardless of how nice she may seem or how you feel sorry for her.
IL's wanted daughter overnight, despite our own hesitations we allowed it, it ended up becoming a regular occurrence. Whenever DD returned, there was something off that we could never put our finger on. We assumed she was overtired.
It transpired, MIL and FIL gave her baths and she didn't like it. It made her and us uncomfortable. We asked them not to, yet did not want to hurt their feelings. They assured us they would stop and made us feel so terrible and like we were the unreasonable ones.
Several years passed until both were arrested for the sexual abuse of DN who had confessed to SIL. It came about that they would do it in the bathroom. They had touched our daughter too but she was spared the same level of abuse he was exposed to.
We have nor ever will forgive ourselves as we let our own feelings and the need of our daughter's fall to the wayside. Today its only a banana but it's still a boundary and it has still been broken.
Speak up and put your daughter first. She could have gone into anaphylactic shock, in that case no one on here would have been saying it's only a banana.