Kindly elaborate as to what kind of affidavit is it?

If your brother is in Jail then you can put an affidavit with your name however if the affidavit puts the name of your brother, you cannot sign it. it is not permissible. furthermore, in the contents of the affidavit, it can be mentioned that you are making the affidavit on behalf of your brother for a specific reason to be given in the contents of the affidavit.