Six years ago, my son was married to the daughter of my sister. They have had 2 sons after the marriage. Now an old woman of the family has told me that once in the absence of my wife, my sister suckled my son. I confirmed this with some other women of my family as well. My question is about what I should do now? Whether this marriage, which was performed in ignorance, is valid or not? What will be the parentage of the children?
At the time of marriage, it should be checked thoroughly whether there is any relationship between the would-be groom and the would-be bride that prohibits the marriage. As you have mentioned in the question, when it has been confirmed that the husband and the wife are brother and sister to each other by virtue of a nursing relationship, then such a marriage is called a Foul Marriage (Nikah Fasid) in the terminology of the Shariah. The children born of that wedlock will take their lineage from the father as the marriage was performed in ignorance.
Now, it is necessary for your son to articulate: I end my relationship with so and so (the name of the wife). Or he should say: I have left so and so. Or the wife should say: I have left so and so (the name of the husband). After the Iddah, your niece, can marry somebody else.
As mentioned in Radd Ul Muhtaar, Kitaab Un Nikaah.
And Allah Most High knows best.
Mufti Hafidh Syed Ziauddin Naqshbandi Qadri
[Professor, Islamic Law, Jamia Nizamia,
Founder-Director, Abul Hasanaat Islamic Research Center]