Is it permissible to marry the sister of a girl with whom I had an emotional and incomplete sexual relationship?

The relationship you had with the aforementioned girl is a sin that necessitates sincere repentance, characterized by remorse, cessation, and a firm resolve not to return to it, so that Allah may accept your repentance. What you did does not prevent you from marrying her sister. However, you should choose a religiously committed wife, for the Messenger of Allah, peace and blessings be upon him, said: "A woman is married for four reasons: for her wealth, for her lineage, for her beauty, and for her religion. So, succeed with the one who is religious, otherwise, you will be a loser." You should also distance yourself from environments that might lead you and the girl back into that forbidden relationship.