If your relative stays at home without contact with anyone from the outside world, they have zero risk of getting the disease. If a relative living in the house needs to leave daily they risk exposure and getting the infection. The relative at home now has a non-zero risk of getting infected.
If the relative at home is independent then they should be left to live alone, and the working relative move in with someone else. If that's not possible, then another person not known to be infected should move in with her, and the working relative moves out.
If the working relative is the main carer for the relative at home, and the above choices are not possible, then they have to manage the situation as though they already have the virus.
The recommendation is that you are not going to transmit the infection to someone who remains at a 6 foot distance away from you, and if inside that distance for less than 10-15 minutes. So, that stops viral droplets from one person passing to the next.
Viral nuclear droplets form when aerosols develop, eg flushing the toilet, so the toilet lid needs to stay down when flushing, and keep windows open. Even better still use different bathrooms, or build a composting toilet away from the bathroom shared with the other person. If you're not able to not share bathrooms, then all the surfaces needs to be wiped down with .1% solution of bleach after each use of the bathroom, and left to dry for 10-30 minutes. Each door knob needs to be disinfected. There can't be any shared eating utensils. And most importantly the hands need to be washed for a minimum of 20 seconds using a soap/detergent.
Even if people live together, studies in China showed that cross infection was not inevitable, but closer to 20% of households had transmission.