Once someone in a house has covid-19, is it basically inevitable that everyone else in the household will contract it (or already has)?

Not inevitable: Hello! If someone in a house has COVID-19, there is some possibility that the other members of the household might get infected. However, that is not the rule. In this case, in order to reduce the possibility of getting infected, the patient needs to be isolated in a room with a face mask, surfaces should be cleaned with disinfectants, and regular washing of hands should be promoted.