My suggestion would be to leave the mother out of it, she's not part of the band and shouldn't have to do the hiring or firing. A little heart to heart talk may help but from what you say I feel this young female has an attitude problem, maybe it's not the type of music she's really interested in, or maybe the little bit of success has gone to her head and she feels that its alright to behave like a hollywood starlet. I also feel that if the shoe was on the other foot she would send you packing poste haste. Good luck