Just say, "Mom, the doctors say I have just 6 months to live." She will say, "What?!?!?" And you say, "Just kidding. I am polyamorous."
But seriously, judge what may work better. Since they are open minded, you can tell them without anyone else present. That will help them ask more questions. If they were closed minded, I would say that you should have them with you as a unified front.
You can probably just bring it up like, "Have you ever heard of polyamory?" And go from there.
When I told my mother, she just said it was an experimental phase I was going through.
I wish you luck.