I have kissed my wife's sister while she slept a few times.
Once she woke up because I had gotten brazen and managed to slip my tongue into her mouth. We never talked about it... I acted like I was trying to wake her up and planned to play it off like maybe she had dreamed it or something, but she never said anything... we just talked. It was so awkward. I knew she knew. Nothing came of it... good or bad.
I wish I would have had the nerve to ask her what she thought about it.