Is the marriage of the brother to his maternal uncle's daughter considered a will after the death of the father who preferred it, and does not marrying her mean the father's anger upon him?

If no harm will result from your brother marrying his maternal cousin, and he believes she will help him remain chaste, then let him marry her as an act of piety towards his parents. However, if he fears that his heart will remain attached to another woman he loves, then let him marry the one he loves to safeguard his religion and prioritize obedience to Allah over obedience to his parents.

While a father's desire for his son to marry his maternal cousin is not an obligatory command to fulfill, fulfilling it is considered an act of filial piety, especially if he has already spoken to the girl's family about the matter, as this would be an enforcement of his pledge. This is further confirmed if the mother also desires this marriage.