What is the ruling on kissing a woman who later turned out to be the questioner's sister in a dream, and are there any legal implications as a result?

What occurred in terms of seclusion (khalwa) and kissing is a sin that necessitates repentance to Allah. One must distance himself from this girl and avoid being alone with her, due to the Prophet's (peace be upon him) saying: "No man is alone with a woman except that a mahram is with her," and "No man is alone with a woman except that Satan is their third," and "For one of you to be stabbed in his head with an iron needle is better for him than to touch a woman who is not lawful for him."