announcing to family

I am 4 weeks 5 days and our original plan was to wait until Thanksgiving to announce to our families that I am pregnant. We are hosting thanksgiving and our parents and siblings will be there and our ultrasound is scheduled 2 days before thanksgiving. The problem is, we don't know if we can wait that long anymore! We're both so anxious to tell people. Also, I work with my husband's sister so if I start showing signs at work (morning sickness for example) I might end up telling her before thanksgiving anyway. My husband wants to tell my dad and stepmother tonight. I'm not against that, it's just that I was planning this big thanksgiving surprise and now I need to come up with something else... not sure how to do it. Ideas??