From what I heard about unicorn triads (someone joining an existing couple to be with both) it's actually the most common scenario that she'll get closer to the husband than the wife (although I guess sometimes it might be the other way around).
Talk about it a lot, but I don't see why you have to walk away. If you do, it's 3 hearts broken instead of one. I don't see why the triad couldn't turn into a V, where you'd be the husband's girlfriend but not the wife. "Symmetrical" isn't the same as "fair", and it would be certainly unfair to demand you break up when you love each other, just because you don't have the same connection with your female friend as you do your male friend.
Definitely be honest with her, as you don't want to hurt her more than necessary by delaying it, leading her on or making her feel left out. However it's possible to recover from it and end up in a different relationship configuration.
Or did they say it's both of them or neither? That seems like a bad plan to me when feelings are involved.