Should the two brothers help their third brother build his house, given that their father gave them two houses and they used to assist him financially before their marriages?

Legally, a father is required to be equitable in giving gifts and grants to his children, based on the Prophet Muhammad's (peace be upon him) saying: "Fear Allah and be just among your children." However, it is permissible to allocate a specific gift to some children if there is a compelling reason, such as their filial piety, a fine they incurred, or a good deed they performed. Therefore, it is permissible to allocate the two houses to you due to your filial piety. You must obey him in his request for help to build a house for your third brother if you are able to do so. However, it is not obligatory if it leads to a deficiency in your essential needs.