Nerd King said:
Ok - faith powers work with up to 10 Corruption - as established in the Inquisitor's Handbook. The sisters should be able to tolerate a *little* corruption before losing their gifts - otherwise there'd be a lot more Repentia.
The issue isn't having a little Corruption. The problem is that she doesn't actually have any faith at all. She's a fake. And assuming that faving the faith talent requires having faith...
It's your game though.