Is the brother's repayment of the stolen money in installments permissible or forbidden for the sister?

It is permissible to demand from one's brother what he stole, and to take it back either in a lump sum or in installments. There is no obligation to forgive him, especially if he is boastful with the intent to cause harm. However, forgiving him and treating him kindly is better, based on the Almighty's saying: "Repel evil with that which is better, and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend. But none is granted it except those who are patient, and none is granted it except one having a great portion [of good]." And according to the Prophet's (peace be upon him) saying: "Charity does not decrease wealth, nor does Allah increase a servant who forgives [those who wrong him] except in honor, and no one humbles himself for the sake of Allah except that Allah raises his status."