What are the rulings concerning kinship, privacy, and interaction between a brother and his sister after their repentance from the fornication that occurred between them? Is marriage permissible between their offspring? And does kinship between them cease to exist?

The act of incest with a sister is terrible, among the most repulsive acts, and indicative of a perverted nature. It is the most severe type of zina (illicit sexual intercourse) and becomes even more heinous if it occurs during Ramadan. However, if you repent with sincere repentance, Allah forgives sins.

If your sister was compliant with you, then she has no claim against you. If you coerced her, then you must seek her forgiveness, unless you fear a greater harm. In this case, supplicate for her extensively. As for what occurred before puberty, there is no sin in it.

If you fear that you might fall into indecency with your sister again, then it is impermissible for you to be alone with her or to look at her. She must treat you as a stranger and observe hijab from you.

If your sister commits zina with someone else, there is no sin upon you, for "every soul, for what it has earned, is pledged." Committing indecency does not affect kinship ties; she remains your sister, and you are an uncle to her children. Marriage between your respective children is permissible.