Unusual for Schizophrenia to set in so late, but it happens. There's also Capgras delusion, where you think your family has been replaced by imposters. Maybe that's one you could point out to her, because it sounds like she doesn't think that ALL the time, so she must know there's something wrong about it. That might be a way to at least get her to see someone about it. Can you mistake your family for impostors? - HowStuffWorks