I found that odd too, and wondered how the BIL knew their schedule that far back. If the victims really were partying with people in the middle of the night on Tuesday with alcohol and potentially other mind-altering substances, and there was a domestic dispute during the party, it does make one wonder what kind of people they were hanging around, and if any of them could have been dangerous / unhinged enough to murder them.
He described the person both as a party attendee and living in the house!