Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The practice of marrying to Maharim is very repugnant among Muslims since its prohibition is well known in Islam. If one does so after having known its prohibition, he may commit Kufr. It is narrated in a Hadith that "The Prophet (Sallallahu Alaihi wa Sallam) sent the uncle of al-Bara bin Azib to behead a person who got married to the wife of his father after his father’s death". [Al-Nasa’e and others, Sheikh al-Albani considered it as sound]
No doubt marrying an uncle is strictly forbidden since he is a Mahram. Therefore, the husband and wife should immediately separate from each other. The woman should have cut this relation when she became aware of the ruling even if her family and husband did not agree. No one should be obeyed who encourages disobedience to our Lord Allah.
Moreover, those who were involved in this marriage, the Wali, witnesses, husband, wife and others should make sincere repentance for this great sin.
The children are traced back to the father since it is considered as a “doubtful marriage” in Islamic jurisprudence, if they had been born before the knowledge of husband; if they were born after his knowledge they should not be traced back to him but they are traced to their mother.
Allah knows best.