What is the ruling on zinā between a brother who is 12 years old and his sister who is 14 years old, knowing that the sister has reached puberty but the boy has not, and is this considered zinā despite their sincere repentance?

Adultery is one of the gravest major sins, and its punishment is one hundred lashes. Adultery with close relatives (incest) is even more forbidden, and its punishment is more severe: execution. Whoever has committed adultery must hasten to sincere repentance, increase in good deeds, and maintain Allah Almighty's obligatory duties, for one who repents from a sin is like one who never sinned. Allah Almighty says: (And whoever does that shall receive the punishment. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace; Except for those who repent, believe, and do righteous deeds, for Allah will change their sins into good deeds.)