What is the ruling of Islamic law regarding the eldest brother demanding a double share of the house that the father gifted to the son responsible for the care of the family and the house, after 15 years have passed since this gift and the son's contribution to building the house and caring for his parents and siblings?

If the construction of the house was a gift and a donation from you to your father, then the house belongs to him, and he must be equitable among his children in giving, based on the Prophet’s (peace and blessings be upon him) saying: “Fear Allah and be just among your children.”

However, if it was not a gift, then you have a right to the house proportionate to what you spent, and no one has the right to object, and the living do not inherit.

And if what you spent on your siblings or paid for their debts was not a gift, then you have the right to claim it. We advise you to fear Allah and to reconcile among yourselves.