What is the ruling of the Sharia regarding someone who, in his youth and out of ignorance, engaged in preludes to zina with his sister, but without penetration, and then repented and sought forgiveness? Is there any expiation or restitution required for that sin?

A child before puberty is not held accountable, and what he commits of prohibited acts is not recorded against him, based on the Prophet, peace and blessings be upon him, saying: "The pen is lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane until he regains his sanity." However, he is rewarded for acts of obedience. Guardians must prevent the child from forbidden acts and accustom him to acts of obedience. If a person finds himself having desires towards his sister, he should distance himself from her, and she should cover herself from him. Marriage or fasting is recommended, as the Prophet, peace and blessings be upon him, said: "O young men, whoever among you can afford to marry, let him do so, for it is more lowering to the gaze and more protective of the private parts. And whoever is not able, let him fast, for it will be a shield for him." It is also recommended to fear Allah and to increase in prayer and remembrance of Allah, for Allah Almighty said: "Indeed, prayer prohibits immorality and wrongdoing, and the remembrance of Allah is greater."