You invite your fiancee to join in

You turn to face Lily, and you meet her eyes. Her expression is hard to read. You can tell she's more than a little uncomfortable, but her cheeks are also flush with arousal. You speak to her just loudly enough for Rachel to hear.

"This pretty young lady and I are going to have some fun. Want to join?"

Lily looks from you to Rachel and back. Then she takes a deep breath and manages a smile as she nods.

"Attagirl, I want you to enjoy yourself too," you say, taking Lily's hand and squeezing it. Then you turn back to Rachel, who is looking at pair of you with unbridled lust.

"On your knees, ladies," you announce.