How can a young man persuade his family to agree to his marriage to a committed, niqabi girl, whom they refuse him to marry because she is not from their tribe and had previously removed her niqab?

If the girl's righteousness becomes clear to you, then strive to convince your parents to marry her through mediation. If they do not agree, then seek out other righteous women whom they would approve of. However, if abandoning the marriage to her would cause you definite harm, then you are not obligated to obey them in that, and you would not be considered disobedient.