How should I let my wifes sister know I want to have with her?

I have been attracted to her since we met, she is so hot, she and my wife look so similar, but she is like 5 years younger, but oh, she is all I want, if I could have my pick of who I could fool around with in the whole world, she would be it. She Has flirted with me in the past, but I am so afraid to approach her, and I think if she knew it would be discreet, she would, what do I do?