What is zina (fornication/adultery)? And is it permissible to marry the twin sister of a girl with whom the questioner almost committed zina, knowing that she is married and talks to him?

The major fornication (zina), which necessitates the prescribed punishment (hadd), is sexual intercourse involving penetration in the vagina or anus of a foreign woman. As for foreplay and preliminaries, they constitute a minor and forbidden form of zina. Sincere repentance is obligatory for what has occurred, and one must turn their heart away from attachment to the married foreign woman and avoid meeting her. It is forbidden to seek to divorce her from her husband. It is permissible to marry her sister if she is a woman of religious commitment and good character, and if there is no legal impediment, bearing in mind that she is a foreign woman before marriage, so it is impermissible to be alone with her or to look at what is not permissible to see of her.