How should I "come out" to my mom about being bisexual and polyamorous?

My fiance and I are polyamorous and have a girlfriend whom we are very close to. Yes, she is *our girlfriend, not simply his, hence the part about bisexuality. I want to be able to talk to my mother about our relationship, but I'm not sure how to tell her I'm bi, let alone that we're poly. She knows that this girl is a part of mine and my fiance's life, but she doesn't know how significant a part. I wont see mom face to face until after Christmas, but I talk to her every night and I just feel like it might not be a good idea to wait that long to tell her. I'm not sure what to do, or what to say. Any advice?

We've told my fiance's father and stepmother, and they didn't really say anything, aside from letting us know thfat they'll make sure there's room for the three of us for Christmas, so they seem accepting enough. Not sure about my mother though.