What should I do about the remaining installments after the death of both the Christian seller and my brother who used to deliver the installments to him?

You must search for the family of the indebted person to pay them their inheritor's installments. If that is not possible, then donate the remaining installments to the poor and needy. Any money whose owner is unknown, whether it be usurped, entrusted, ill-gotten, or otherwise, should be given in charity and spent for the benefit of Muslims.