Is it permissible for a girl to marry against the refusal of her guardian (her brother) who deems the suitor and his family unsuitable? And is it valid for her mother's husband or one of her paternal uncles to be her guardian in this case? And does the brother incur sin by refusing to marry off his sister to someone he deems unsuitable for her?

The guardian is a condition for the validity of marriage according to the majority of jurists. A guardian's rejection of a suitor falls into two categories: either there is a legitimate Sharia reason for it (such as a flaw in the suitor's religiosity or character), in which case he is permitted to refuse; or there is no legitimate reason (meaning the suitor is upright in religion and character), in which case the guardian ought not to reject him. If he insists on refusing, the girl may refer her case to the Sharia judge to marry her off or appoint someone to marry her off. The mother's husband is not a guardian unless he is among the 'asaba (agnates).