Okay, so apparently Samantha's vomiting last week wasn't a food thing; it was a bug thing.
Sam and I left Hannah home in charge of Rachel and Samantha while we ran 5 miles down the road to pick up Abby and Nathalie. We arrived home just as Hannah was throwing up in the kitchen! She was fine when we left! Honest!
I cleaned the kitchen, floor, walls, cabinets, and refrigerator, while Hannah took a shower. Abby cleared off Hannah's bed, and Nathalie and Sam put Rachel and Samantha to bed. Hannah continued to throw up through the night, just like Samantha had six days ago. So we figure for the next 3-4 weeks, every 5-6 days, we'll have someone getting sick. Something to look forward to.