Am I entitled to keep the remaining amount (12,0 Swiss Francs) of the money allocated for my brother's treatment, which is in my possession, to use for my marriage, or must I give it to my brother?

The money given to a patient is their property, and it is not permissible to seize any part of it unless what you spent on their marriage was intended as a loan, not a donation or a gift. If you spent that money as an act of charity and benevolence, then it is not permissible for you to reclaim it from their funds, based on the hadith: "The one who takes back his gift is like a dog that returns to its vomit." If the giver and the recipient disagree, the statement of the giver is accepted, provided they take an oath.