If you are of legal age, you meet certain other criteria, and you are able to support your brother, you can file a Petition for Guardianship over your brother. You will need to be able to show that your mother and your brother's natural father are unfit, unwilling, or unable to take care of your brother and that it is in the child's best interest for you to have guardianship.
I advise you to consult an attorney in your area who practices in the area of family law.
This answer is not intended as legal advice but is educational in nature. The choice of a lawyer is an important decision and should not be made on the basis of advertisements.
