What is the ruling of Islamic law on someone who fondled his sister in a manner akin to fornication, without penetration, when neither of them comprehended the matter, and his age was 14 years and hers 10 years?

Allah has forbidden zina (unlawful sexual intercourse) and its مقدمات (preliminaries). The sin is compounded if it is committed with a mahram (a person with whom marriage is forbidden due to kinship). If they were adults, they must repent to Allah for this heinous act. However, there is no sin upon them if they had not reached puberty. Parents should separate their children in beds at the age of ten and keep them away from anything that could corrupt their morals.