Will the young man be sinful if he marries a girl despite his parents' refusal due to differences in nationality?

If the girl is righteous and her family rejects her due to a difference in nationality, then it is not obligatory to obey them in abandoning her marriage, and there is no sin in disobeying them. However, it is advisable to leave her and seek another woman whom the family approves of, if there is no harm in doing so, because a shared nationality may foster understanding and harmony between the spouses.