I don't think the craziness is entirely on Mia's end - Ch20 implies that Sara went into Mia's room, tore up her doll, and put sooty handprints all over the room.
My guess is that Sara for whatever was a bit mentally unstable, and that instability influenced Mia via the coffee brainwashing effect, which then influenced Sara herself because humans mentally influence their shadows, and so now you've got a very dangerous feedback loop where they increase each other's craziness.