Must a son return to all his siblings the lands that were registered to him by his father, or do they remain his, or what should he do, especially given the presence of siblings from another mother and the claim that these lands were for the support of his full sisters?

The father must be just among his children in giving gifts, whether they are males or females. Preferring some over others without a valid reason is an injustice. If the father singles out some children or differentiates between them without a legitimate reason, he is committing a sin and must repent and return what was given extra to some or give the others what completes their share. If the adult, mature siblings agree to what the father wrote for his children, there is no harm in that. Otherwise, all of the father's estate must be divided, including what was written for some children without a valid reason. As for what the questioner spent on his siblings, he may claim it back from them and take the amount he spent if he did not intend it as a donation. Scholars have stipulated that the one who spends may take back the amount he spent if he intended to claim it back.