What is the ruling on Ali marrying Zaynab, knowing that Zaynab was suckled by the wife of Ali's paternal grandmother, and Ali is the son of his paternal grandmother's granddaughter from her maidservant? And is it permissible not to inform them if the marriage is unlawful?

The milk of a nursing mother affects the suckling and their offspring. The suckling is considered a child of the nursing mother and her husband, and the relatives of the nursing mother become relatives of the suckling through nursing.

So, if "Zaynab" was nursed by "Amina," "Zaynab" becomes the daughter of "Amina" and her husband through nursing. Consequently, the daughter of "Amina's" husband (Halimah) becomes "Zaynab's" sister through nursing, and Halimah's children (such as "Ali") become mahram to "Zaynab" because she is their maternal aunt through nursing. Therefore, their marriage to her is impermissible.

If "Ali" has married "Zaynab," this marriage is void and must be annulled immediately, because "Ali" is considered to be married to his maternal aunt through nursing. As for the children resulting from this marriage, they are legitimate and are attributed to their father "Ali," because the intercourse here is considered intercourse based on a misconception (wat' shubhah).

The involved parties must be informed of this, as it falls under the category of offering advice and changing what is wrong.