I've never used IEMs as a musician or a vocalist, so take that into consideration before accepting what I have to say. Anyway, I would say that on a loud stage where the vocalists need monitors to hear each other, using IEMs vs wedges should not make a difference. Using them right, it can actually help protect their hearing in the long run. Now, if the stage is quiet and the monitors are only needed to give the singers a better reference for rhythm and pitch, then it could take some time for them to get used to them, but in the end, since you should be using pre-fade mixes for them, they should be able to blend just fine.
As for your current wedges, with only two, you shouldn't really have a problem with them washing into the audience unless they are simply too loud, or your space is too small. You only need to put through them things that are difficult for the users of the wedges to hear. So, if you have an acoustic drum kit and guitar amp on stage, don't put those in the wedges. If you have electronic drums, and guitar, bass, and keys are all going direct, then you might run into the problem of having too much in them for the vocalists to pick out what they need, and they in turn ask for either "more me" and it cycles into a louder and louder mix.
I do think there is a tendency among some to go a bit too far in reducing stage volume to the point where it can seem less "real" or more unnatural, especially to those with a classical background. Since your band leader appears to fall into that category, that might be more of the reason for her resistance to use IEMs than the ability for the vocalists to blend.