Can the brother marry off his sister (i.e., be her guardian) in the presence of the girl's paternal uncle? And who are the 'Asaba (agnates)?

The 'Asabah (agnates) are those who inherit by way of ta'sib (agnatic inheritance), and the guardian of a woman in marriage is one who inherits by way of ta'sib. The closest in proximity is given precedence over the more distant in guardianship. So, if the more distant guardian performs the marriage while a closer one is present, without a valid excuse, the contract is invalid. A full brother and a paternal half-brother have more right to give a woman in marriage than her paternal uncle. As for a maternal half-brother, he does not have guardianship in marriage because he is not among the 'Asabah.