Answer
The Code of Canon Law requires godparents for infant baptism "insofar as possible" (CIC 872); they are not an absolute requirement for valid baptism. To be a godparent, a person must be one "who leads a life of faith in keeping with the function to be taken on" (CIC 874 §1). Your brother and his wife do not qualify because of their irregular marriage. In such a case, their sponsorship as godparents would be illicit (not in conformity with the law of the Church), but the baptism would still be valid.
