Is it permissible to accept and utilize a farm built by a brother with ill-gotten money (stolen from the state), which he then beautified with an usurious loan?

If your father bought the farm with mixed money or with an interest-based loan, the same ruling applies to the shop he bought with mixed money. Some scholars have permitted dealing with someone who bought with the very stolen money, based on the premise that the unlawful act pertains to the seller's responsibility, while the buyer owns the item lawfully. Therefore, the farm your father bought with mixed money or with an interest-based loan is lawful for you to benefit from.