All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Sitting in seclusion with this non-Mahram woman and kissing her is a sin that necessitates repentance to Allaah The Almighty. The Prophet, sallallaahu ‘alayhi wa sallam, said: “No man should meet a woman in seclusion except in the presence of one of her Mahrams.” [Al-Bukhari and Muslim]
He also said: “No man meets a (non-Mahram) woman in seclusion but the devil is their third (companion).” [At-Tirmithi: Hasan Saheeh]
In another Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, said: "For one of you to be stabbed with an iron needle in his head is better for him than touching a non-Mahram woman." [At-Tabaraani; Al-Albaani: Saheeh]
You should repent sincerely of this sin. You should also praise Allaah that it stopped at this point and did not become worse than that.
It should be known that your feeling of the change of the girl’s face to be like the face of your sister is out of the Care of Allaah for you. So, praise Allaah for having saved you from Fitnah. To show complete gratitude, you should keep away from that girl and beware of being in seclusion with her.
Allaah Knows best.