I think, it was definitely the same day, when little W disappeared. (I remember my own lack of understanding for the situation. I thought at the time, the FFC got some privileges, which another person wouldn't have been allowed to have.)
Meanwhile I'm wondering: Did the sister arrive at the (unknown?) airport OR was she already at her mother's home before? If a sister was already in Kendall, then it must have been this sister, because FFC has only one sister, afaik. Not even that is clear; it is exasperating.