Is it permissible, Islamically, for a person to receive his military salary without working, and is this money considered lawful or unlawful? And is it permissible to give it to his orphaned brother to spend on his studies if he refrains from spending it on his family?

It is not permissible for a person to take a salary after leaving work and discontinuing attendance, nor is it permissible for him to benefit from it for himself or his family, or to give it away in charity. Rather, he must return it to the employer if he knows that it will be spent appropriately, and inform them of his absence from work, in accordance with the Almighty's saying: "And do not consume your properties among yourselves wrongfully." He must return whatever salaries he received after leaving work. If he knows that the governmental entity may not spend those funds appropriately, then he must spend them on general Muslim welfare or on the poor with the intention of divesting himself of them, not with the intention of charity.