Something about glass houses....
Something about glass houses and stones comes to mind here. From what I've experienced many people from the so-called "normal" world would have a a squick and or gut check about most anything they read here. The concept of poly seems to be beyond the closed minded Victorian upbringings most of the western world is brought up with.
I may be a newbie, but I think that if you care for both of these women, then communication is the only answer. But remember that blood is thicker than water. Relationships come and go, but family is forever.
Have you looked inward and examined your motives? Do you really and truly care for them both or could it be a forbidden fruit type of thing?
I imply nothing, I'm only playing devil's advocate.
Anything worth having will take a great deal of effort!