To address Kendall's slant on the situation, in my opinion it shouldn't make any difference whether the singer (or other musician) in question is male or female.
And I agree with those comments posted about taking this directly to the 18 year old and leaving Mummy out of the picture, and having the confrontation/discussion privately, calmly and away from the other band members.
If your concerns had not been addressed to this person prior, I'd give that person one chance and one chance only to right the situation (assuming they want to remain in the band, and assuming they're not making you feel like they're doing you a favor staying.) If however any further work is lost or jeopardized as a result of this person's on stage or off stage behavior or attitude, with any affected third party (other band members, club owner, or audience members) they should be terminated immediately.
If I were "GUEST,A. Noni Mouse", I would quietly and very discretely start looking for a replacement singer now, as a precaution, and, to check out your options and to see who else is out there, available, appropriate and enthusiastic about joining your group.
Never make any one member of your musical group or band, feel like they are indispensible, for all of the obvious reasons.