Would a person be sinful if they financed the construction of their father's house, which uses usurious loans for its construction, by covering the costs of materials such as cement, iron, and sand?

It is not permissible for a Muslim to borrow with usury for building a house or for any other purpose, except in a case of extreme necessity, which is when he cannot find housing for himself and his dependents, even by renting. Whoever borrows with usury without necessity must repent to Allah. As for purchasing building materials for his father with that loan, there is no objection to it; because the sin is tied to the borrower's responsibility, not to the money itself, and this is not aiding in sin. The brother should advise his father to repent and explain to him the impermissibility of borrowing with usury.