Backup? or attention seeker...?

So i'm in this band called Define Authority and i'm the lead singer. But thats not the problem, the problem is my friend, who is a backup singer, is very loud. No one can hear themselves play. She is an alright singer but she is overpowering everyone else. http://www.youtube.com/watch?v=wgyF2Rm6hDg theres thhe link to me and her singing so you can get an idea of her. I sing the "i could use a dream or a genie or a wish" she sings other bits. this was edited on garageband so its not a fully idea. but anyway, how do i tell her that she sounds too loud and it doesnt sound very nice. She will hate me forever if i tell her because she thinks she is very good. And she is offkey too. Im trying not the be selfish, in fact, its because of the other people im doing this. No one can hear themselves and it is stuffing up everything. Plus she is backup and yeah.... And dont say, if she is a real friend she will understand, cuz she takes everything to heart. we tried turning down her microphone a bit but she got angry and left the room. so please, how do at least HINT that it sounds kinda annoying