They’re thinking too simplistic.
Ebola spreads due to it shedding in all body fluids but it invades and saturates different systems at different rates. Apparently, Duncan had a low concentration in his sweat and other fluids that he left in that apartment.
Nurses get it because end stage Ebola is systemic. Near the end it’s in all fluids in high concentrations.
Higher concentration... Higher risk.
The family got lucky.
It’s possible that members of his family DID get ebola, but only in a very mild form (for some reason this happens with some people).
IMHO they should ALL be checked for anti-bodies; and if these are present, then their blood might be used to help those with worse infections.