[M4F] Help me convince my wife to let me fuck her sister

My wife has recently opened up to the idea of letting another woman watch us in the bedroom with the goal of eventually including her once my wife is more comfortable. I’m excited, but to be honest strippers and hookers don’t really do it for me. The only woman who’s body drives me absolutely crazy is my sister in law’s. I’ve spent more nights than I can count over the last few years fantasizing about her tight, young ass while fucking my wife.

We’ve flirted a few times, she’s straddled me once during a party as a joke, and she’s a total perv. We often talk about hentai and nsfw games, but I’ve been careful not to cross the line. Recently, I bought my SIL a funny thong as a gag gift and my wife thought it was hilarious.

Here’s where you come in. Help me figure out how to include her while being subtle and careful. I want my wife to think that it’s her idea and that I’m actually resistant to it. Also, if you’ve pulled this off yourself before I’d love to hear from you!