Are the two girls planning on playing together? or is it just that you get to play with the two of them? I think that makes a difference on the incest factor. If they are playing together then you are definately dealing with incest.. which most people consider completely wrong and immoral.. then again there were entire countries founded on incest (where the term blue-blood originally came from was long lines of families who "kept it in the family" to keep the blood line pure. The main reason incest is illigal is because of the likelyhood of birth defects/deformities (obviously if the two women are playing there is no chance of that).
I'm not saying it's right or wrong, just giving a little background info.. to each his own. IF everyone involved is cool with it and consenting adults who are we to judge?
But do keep in mind the potential difficulties this could cause in your families if things did go bad or somehow the word got out.
Julie
http://www.swingersboard.com