Is the older paternal half-brother considered a creditor to his full siblings or to their father, in return for what he spent on them in their youth, and must this debt be repaid in the same quantity or at its current value?

What your brother did for your mother by spending on you is a righteous deed for which he will be rewarded, and you must thank him and treat him with kindness. The children's maintenance is obligatory upon the father, and if he abstains, the mother may borrow and claim it back from him. If your father abandoned maintenance and your brother took over, there are two scenarios: First, he spends with the intention of claiming it back from your father, or your mother, or you. This is permissible, and his intention is between him and Allah. Second, he does not intend to claim it back; he is then a volunteer, and he has no right to demand what he spent because that would be a retraction of a gift, which is forbidden except for a father regarding what he gives his child. Therefore, make it clear to your brother that it is not permissible for him to demand anything from you unless he intended to claim it back at the time of spending. If he did intend to claim it back, then what you paid him through your mother should be deducted because it was a reward for his kindness. If he had intended to claim it back, he should be repaid what he spent. Given the currency depreciation, it is more likely to be reconciled by splitting the currency difference between you, such that you look at how much the money was worth then and how much it is worth now, and you both bear the difference together.