I tried to read the thread linked in the OP, and just couldn't get it done. As it meadered thither and yon, a spontaneous SEP field kept rendering it invisible to me. Sorry.
As for sisters sharing spouses: it's been done. The highly publicized example that springs immediately to mind is sisters Leah and Rachel sharing husband Jacob in the Old Testament of the Bible.
(Oh yeah: For those of you who don't keep your copy of the Hitchhiker's Guide to the Galaxy with you at all times, an SEP field generator is a very useful device that can render almost anything completely invisible to most sentient beings by making them believe the thing in question is simply Somebody Else's Problem.)