This is why I wouldn't want to work for family. I understand that your mom is a great mentor and resource for networking, but I don't think people who work for their parents get as much respect. At least that has been my experience in 2 offices. :shrug:

If I were you, I would talk to her personally and settle it yourself in a calm, adult manner. Tell her that her shift ends at 12pm Wednesday so if anything that happens before 12 carries over past 12, she is required to stay and finish the task. Tell her that you were required to do it when you were in that position (so you don't seem like you're trying to be authoritative).