Are the parents entitled to prevent their son from marrying a girl he loves and who wants to marry him, due to some feelings of love from his brother towards this girl (while the brother does not wish to marry her), and due to the parents' dislike of the girl's family because of their bad morals?

If the girl is righteous and pleasing in her religion and character, then the parents should not prevent their son from marrying her. The son should try to persuade his parents through supplication and by seeking mediation from people of good character. If he is unable to convince them, then the scholars have stipulated that obedience to the parents takes precedence over the desire to marry a specific woman, unless the son fears falling into sin, in which case he should prioritize what helps him maintain chastity over obedience to his parents. And if the son foregoes his desire and marries someone else out of dutifulness to his parents, then Allah may bless him in that marriage.