What is the ruling of Islamic law regarding a son's insistence on marrying a girl rejected by his family, who threaten both him and her, especially given that the girl has insulted the son's mother?

It is not permissible for a child to disobey his parents in what is considered forbidden disobedience, and this includes marrying a girl whose marriage would cause them harm, as harming parents is forbidden by the Almighty's saying: "So do not say to them, 'uff'" (Al-Isra: 23). The child must fear Allah and observe His commandment concerning his parents, and beware of disobedience, which is one of the gravest sins. If he insists on his course, then he should be forgiven, and what he has done should be blessed and he should be helped.