Is a father religiously sinful for having given one of his two homes as a gift to his eldest son 13 years ago? Is it permissible for the siblings in the future to object to this gift, or to demand that the father ask his son to pay the price of the home or revoke the gift?

We hope that your father incurs no sin for what he did, because he singled you out with the house due to your specific need for it. This is what some scholars have stipulated regarding the permissibility of singling out some children if there is a valid reason, such as need, or chronic illness, or a large family, or preoccupation with knowledge, or the immorality of another son. Ibn Qudamah stipulated this and cited what was narrated from Ahmad regarding the permissibility of singling out due to need. Preferential treatment is also permissible if the children agree. What indicates this is what was narrated from Abu Bakr and Umar - may Allah be pleased with them - that they granted gifts to some of their children without others, with the consent of the remaining children. You mentioned the siblings' consent to what the father did, so it is a valid and effective gift, and no one has the right to object.