From what I've read, it seems like she wasn't 100% confirmed to go to mom's for dinner and often she didn't end up going. They discussed what mom was making but she just responded "ok" to hearing what it was, no confirmation either way for sure. It seems like her mom thought it was pretty normal to have discussed dinner but for her to not turn up in the end. Her mom knew she was alone at the house but it seems like that also probably wasn't that out of the ordinary given the work her boyfriend and the others in the house do that probably takes them out of town frequently. This is all just my understanding, the details are scarce.