Is the female questioner sinful if she stops advising her brother about his relationship with a girl, after advising him for a year without success?

Advising, enjoining good, and forbidding evil are among the rituals of the religion, especially towards relatives. Whoever undertakes this must be patient and have good faith in Allah, and not despair. If a servant fulfills his role in da'wah (inviting to Islam), he is not sinful if the disobedient persists in his disobedience, and it is permissible for him to abandon him if abandonment benefits him. It is advised to be patient with the younger brother and vary the methods of inviting him, and to seek assistance from family members who can influence him.