Love for wife's sister

I love my wife's sister for 10 years but not expressed my love. But however my wife has come to know about it recently. But she is not angry. She wants me to express my love. She says if her sister accepts the love. My wife will be happy but wants that both us can have physical relation but occasionally not on regular terms

So let me get this straight. You've told your wife how you feel about her sister and she's ok with it? What exactly are you waiting for? If the sister wants some, then go get some!!!