What is the ruling of Islamic law regarding a brother marrying a fornicating woman who became pregnant by him and then aborted, despite the family's advice that she comes from a disreputable family?

If the marriage is between two fornicators, it is not valid, because chastity is a condition for the validity of marriage, according to the preponderant opinion of the scholars, who cite the Almighty's saying: (The fornicator does not marry except a fornicatress or a polytheist, and none marries the fornicatress except a fornicator or a polytheist. And that has been forbidden to the believers) [An-Nur: 3]. Therefore, opposing and denouncing this marriage is obligatory, because the Prophet, peace and blessings be upon him, said: (Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart – and that is the weakest of faith).

Moreover, swearing not to attend the wedding or enter her house is legally مطلوب (required/demanded) unless there is a legitimate benefit in attending. If fulfilling the oath conflicts with honoring parents, then atonement for the oath is obligatory, and one must enter the parents' house, because honoring them is one of the most incumbent duties. The brother must be informed of the religious ruling and reminded to repent before marrying this woman.