I kissed my wife's sister

We fell asleep next to each other on the couch watching television. When I woke up, somehow, my arm was around her and her head was leaning on my shoulder. Before I knew it, still half asleep, I leaned over and kissed her lips. She started to kiss back and then a few seconds into it, she woke up. It was awkward and we have never talked about it... it was such an amazing moment. I so want to kiss her again.