Is ghusl obligatory for someone who committed zina with his sister before puberty without ejaculation, and are his prayers and Umrahs valid in this case, both before and after puberty, and what should he do?

You committed a repulsive act, and the sin is lifted from you if you were not an adult at the time. Adulthood is reached by the emission of semen, or the growth of coarse pubic hair, or reaching fifteen years of age. If you did that before reaching adulthood, then ghusl (definition) (ritual bath) is obligatory for prayer and circumambulation (Tawaf (definition)), not for sin, and it becomes a condition for the validity of prayer and Tawaf, and for the permissibility of reciting the Quran and staying in the mosque.

Al-Nawawi said: If a child penetrates a woman or a man's anus, or a man penetrates a child's anus, or a woman inserts a child's penis, then ghusl is obligatory for the woman, and the child becomes junub (in a state of major ritual impurity). The same applies to a young girl if a man or a child penetrates her. Likewise, if a child penetrates another child, whether discerning or not. His prayer is not valid except with ghusl, and the guardian must order him to perform it. If he does not perform ghusl until he reaches adulthood, then ghusl becomes obligatory for him.

Ibn Qudamah said: If the penetrator or the penetrated is young, then ghusl is obligatory for both of them. This does not mean they are sinful, but rather it is a condition for the validity of prayer, Tawaf, reciting the Quran, and staying in the mosque.

Your prayers and Tawaf before ghusl are not valid according to this opinion, and your Umrah (definition) is not complete except after a valid Tawaf. You are not required to make up the prayers due to the absence of accountability before puberty. If you delayed ghusl after reaching puberty, there is a difference of opinion regarding the obligation of making them up.

What you committed of the prohibitions of Ihram (definition) is excused due to your ignorance. Some scholars are of the opinion that ghusl is not obligatory for those who have not reached puberty by the penetration of the glans. Based on this, your prayers and Umrah are valid, and there is no harm for you in adopting this opinion.