Get rid of her. I used to do a two hour set by myself (acoustic guitar and singing every song) that I could easily get through without a break.
WHO needs a break from singing, after only a few songs .......... that's clearly not normal. Even a trumpet player can get through more than 3 songs without needing a break. Trust me, trumpet is 10 times more demanding than singing.
The only reason I could see to keep somebody like that is if she was the one getting you guys all your gigs. Because she's so ......... awesome ...... gorgeous ....... a great sales person. Doesn't sound like that's the case. Kind of sounds like you're taking over her singing anyway.
I can't see any excuse for this ..... Singers absolutely need to work on their voices to keep them in shape. While I was doing those 2 hour gigs, I was also practicing 3-4 hours a day. I'm serious about being a musician. When a singer actually PRACTICES, their voice usually can handle the rigors of gigs quite easily. Laziness doesn't help any band.
And, sorry to be so blunt, but LESSONS are what many, many singers need these days ...... Not just to sound good, but to have easy going voices rather than straining voices. Even Stevie Wonder had to get lessons late in his career to learn his high powered singing style without straining, when he developed nodes on his vocal chords. Lousy singers are the worst. Get some lessons.
End of rant. I feel better.
Turtle