@Mathiason it's clearer in the wn, that Reiko has a long history of having to accommodate delusions, because her grandmother was extremely delusional (and violent) in her last years. Fortunately, the violence was against the maid Fuki-san, but Reiko was stuck in the middle of it because Mary had reverted entirely to speaking only French, which Reiko spoke but Fuki did not.
I'm not marking this 'spoiler' because it has been mentioned once in the manga at this point, it's just made more clear in the wn.