What is the religious ruling on a father gifting his children from his second wife, to the exclusion of their half-siblings? And do their half-siblings have the right to claim that land or the land that was distributed among them?

What the father did is a gift (hiba), and if it was possessed and taken into custody during his lifetime, it is valid. If the donor dies before any modification, the gift proceeds as it is, according to most scholars. If a father favored some of his children in giving gifts and then died, that gift is affirmed for the recipient, and the remaining heirs have no right to revoke it. The gift proceeds as the father distributed it if the children took possession of it during his lifetime, and its form as a simulated sales contract is of no consequence, for what matters are the meanings. If the third brother was included in the gift after his birth, there is no problem. If he was in the womb, there is a difference of opinion regarding the validity of the gift to him; the majority forbids it, while some permit it. Therefore, it is preferable to uphold the father's gift as it is due to the passage of time and to avoid disputes.