Being from Africa they might have an immunity.
Nobody knows exactly why some people get it from being exposed to someone and other people don’t that are exposed to the same person.
I remember reading they know of one group in Africa that has a natural immunity to at least one strain of Ebola.
They might very well be immune, or they were never in the house to begin with. My Mom is immune to chicken pox, never got the vaccine and all of us had the chicken pox but she never got it. She had a blood test and it showed she is immune to it, not sure if someone can be immune to Ebola, I doubt it, which makes me think they were never in the house