Are First Husband’s Sons Mahram to Second Husband’s Daughters?

Why First Husband’s Sons Are Mahram to Their Father’s Former Wife

The sons of your first husband are Mahrams to you , because you were the wife of their father and you are not permissible for them to marry, because Allah says (interpretation of the meaning): {And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way.} [An-Nisa’ 4:22]

Mahram Relationship between First Husband’s Sons and His Daughter

Similarly, the sons of your first husband are Mahrams for your daughter from the first husband, because she is their sister through their father.

Are Daughters from Second Marriage Mahram to First Husband’s Sons?

With regard to your daughters from the second husband, there is no Mahram relationship between them and these sons, because there are no ties through blood, breastfeeding or marriage which would create that Mahram relationship.

Based on that, it is not permissible for them to uncover in front of the sons of your first husband, and it is permissible for them (the first husband’s sons and your daughters) to marry one another.

Permissibility of Marriage between First Husband’s Sons and Second Husband’s Daughters

Al-Kasani said in Bada’i` As-Sana’i` (4/4):

It is permissible for a man to marry a sister of his half-brother through his father. What that means is if his father’s wife gave birth to a son, and she had a daughter from another husband, that daughter is the sister of his half-brother through his father, but it is permissible for him to marry her. End quote.

This is the situation mentioned in the question. It is permissible for the sons of your first husband to marry your daughter from your second husband, who is the sister of their sister through the father.

For in-depth explanations, see the answers listed here:

Who Is a Woman’s Mahram

Etiquette of Talking to Non-Mahram Women

The Maternal and Paternal Uncles of the Husband Are Not Mahram to the Wife

Sitting with Husband’s Relatives and Shaking Hands with Them

Is the Father of One’s Ex-Husband a Mahram

Is My Husband’s Stepfather My Mahram?

And Allah knows best.