Thanks for the reply. I see the problem. Usually when people live close proximity to each other like siblings in childhood, they lose sexual and romantic feelings for each other. This is known as Westermarck effect in psychology. Since you didn't spend time in close proximity on a regular basis during your childhood with your sister you may develop romantic feelings for her.
I am pretty certain that your sister also have feelings for you from the story you described. The best thing is to do is talk to her about the night. After that, act and behave regularly with her. Your feeling will naturally go away with enough time. One good thing may happen is your bond between your sister and you may be stronger after that.