Is the marriage of a brother to the one he loves considered an injustice to his paternal female cousin, whom he had promised to propose to?

Your brother's marriage to this woman is not an injustice to his cousin. However, his marriage to her, despite his parents' rejection, is not permissible if their rejection is justified. But if they reject without justification, then he is not obliged to obey them, and he would not be undutiful to them. If marrying the woman your parents desire would not harm you, then this is an act of righteousness and kindness. Moreover, the relationship between young men and foreign women has Islamic limits and regulations, and what is known as "love relationships" between young men and women are doors to evil and corruption.