Hide in the closet.

You immediately realize you want to enjoy this seen more before becoming involved. So you move into the closet that is across from the foot of the bed, and leave the door partially open. In moments you see your wife and Cindy enter the room from the left, and your wife continues to the bathroom.
"Ooh. That's interesting. Why do you have this mirror near your bed Diane?", Cindy asks. The mirror she mentioned is a full length changing mirror at the front corner of the bed near your wife's dresser.
Diane answers as she comes out of the bedroom, "Its so I can check my outfits when I change, among other things." You notice your wife also has a quizzical look on her face until she notices the closet door ajar and you peering out.
She then moves Cindy slightly over and squirts some lotion into her hands. You look, and see she positioned Cindy so that it affords you a view of them from behind and their reflections from the front. Diane begins to run the lotion into Cindy's shoulders until she states "This just won't do Cindy. Your straps are in the way. Should we remove your top?" Cindy replies coyly, "Yes." And then Diane unsnaps it and slowly pushes it off Cindy's shoulders and lets it drop to the floor.
"My. What magnificently firm and full breasts she has for her age." you think to yourself of Cindy. Next you see Diane has reached over Cindy and is gently rubbing the lotion onto her tits, and begins to cup them and pinch their nipples.
Cindy moans, and rests her head back on Diane's left breast.