I have to agree with the possibility of looking for new jobs, just in case. I am also the type though that although I do not shout about my poly relationship from the rooftops, I also do not try to hide it. To me hiding means I am doing something I think is wrong and it also give others ammunition to use against me.
they could have a sit with everyone including mom and BB and spill the beans in a very simplistic way. "Although it is not something we advertise it is also not somethign we are ashamed of and since you are sort of part of the family now we wanted to share with you that we are a family unit and care deeply for one another."
You know, nothing too in depth. Keep it on the surface and all about the family structure and how that makes them happy and fulfilled. Then be prepared for questions. lol
Or, they could go more along the lines of what I do. I live my life and if someone has a question and they ask it, I answer it. So instead of mom trying to hide it she could say something about how nice it is that the whole family might be getting together for the holidays and open the topic up that way. If the questions end up being of a personal/sexual nature I often will say something along the lines of "I'll answer your private question if I get to ask one about you and your (insert relationship title here)." That usually keeps the questions out of the sexual realm. lol
Anyway, best of luck to them. It really is too bad that people tend to be so closeminded and blind to true happiness when it is right in front of them.