The family members lived for days in a contaminated environment. They'd have to be damn lucky. Unbelievably lucky.
Hope your hypothesis is correct.
They are in day 14 after their last contamination. Maybe only day 12 if you consider that the authorities left them in their apartment for two more days after Duncan was hauled off to the hospital. So they have a few more days until they can be considered very low risk, but after day 10 it was somewhat of a relief since it took 10 days for Duncan to come down with symptoms.