Is the action of the two brothers who forced their father to sign a document that reduces their brother's share in the business and unfairly distributed the lands considered an injustice, and what is their recompense from Allah?

The work of sons with their father in trade has three possibilities: that they are partners, or employees, or volunteers. This is determined by contract or custom.

If they are partners, they have the rights of a partner, and it is not permissible for the father to take his son's money to give it to another, nor is it permissible for partners to infringe upon the money of their partner.

If they are employees, they have their wages. And if they are volunteers, they have nothing.

If it is proven that the two brothers sought to prevent or deprive their brother of his right, then they are unjust, and the injustice of a kinsman is great. They must repent and restore rights.