Is it permissible to inform the mother of the cousin's daughter's desire to marry someone else, given the brother's refusal to marry relatives, and the mother's belief that the sister is the cause of this refusal?

If the brother insists on not marrying the girl his mother wants him to marry, and the mother cannot be convinced except by stating the truth about the girl's wishes, then there is no objection to doing so. This preserves the brother's dutifulness to his mother and absolves the questioner of influencing his brother's refusal.