As revealed near the end, Mother Miranda kidnapped Mia Winters at some point to replace her and apparently played the role well enough that Ethan didn't really ask any questions before Chris came in and shot her without explaining jack shit to Ethan.
Ignoring the question of whether this part of the plan was even necessary, exactly how long was Miranda planning to keep up the act before absconding with Rosemary and (presumably) killing Ethan, considering that she apparently spent about a week doing this before Chris showed up?


