"I didn't tell you to stop."

Your Wife seems surprised by your comment.
Your Sister-in-Law less so.

"You want to...watch me with her," the former asks.
You nod.
"But compared to her, I'm..."
"Beautiful. Lovely. The woman I'm going to be with forever," you interject.

She blushes and starts to cry a bit.
Happy tears, of course.

"If that's what you like...I'm game. Are you?"
Your Sister-in-Law grabs her and pulls her back in for another kiss.

Question answered.

You make sure that the door is closed and locked.

The younger lady sees you watching again and ups the ante.
Her right hand goes to your wife's back...and slides down.

She fondles her ass and your wife is a bit surprised.

She looks at you for a second and her own hand goes to work.
Her small fingers find the young lady's tits under her blouse.

At this point, you are hard as a rock.

After another couple of minutes, the young lady looks at you.
You pantomime pulling off your shirt.