Is My Husband's Son a Mahram to Me?
Question

My wife’s sister married a brother who had two children from a previous marriage. They were babies when she married him and she has raised them as if they were her own. I don’t even know if they know that she isn’t their natural mother. I do not know for sure, however I believe they were never breast fed by her. All of my wife’s family including my wife treat them as my wife’s sister’s natural children. They are both reaching puberty now. I wish to know will my wife have to wear Hijab in front of the boy and will the girl have to wear Hijab in front of me?
Praise be to Allah.Shaykh Ibn `Uthaymin (may Allah have mercy on him) said:
Why Does a Wife Observe Hijab around Her Sister’s Step-sons
Your wife has no connection to the children of her sister’s husband, because they are not her sister’s children, either by descent or through breastfeeding. Based on this, your wife has to observe Hijab in front of this boy because he is a “stranger” (non-Mahram) to her.
By the same token you are a non-Mahram to this girl, so it is not permissible for you to be alone with her or to travel with her, and it is not permissible for her to uncover her face in front of you.
The Mahram Status of a Wife to Her Husband’s Children
With regard to the wife of the children’s father, she does not have to observe Hijab in front of them because they are among her Mahrams. Allah says (interpretation of the meaning):
{And marry not women whom your fathers married, except what has already passed.} [An-Nisa’ 4:22]
Islamic Ruling on Mahram Relationships in Extended Families
This verse means that it is not permissible for a man to marry a woman whom his father or grandfather had married, no matter how far along the line of ascent, regardless of whether the father or grandfather is through his mother’s line or his father’s, and regardless of whether the marriage was consummated or not.
If a man has entered into a valid marriage contract with a woman, she becomes a Mahram for his sons and grandsons through his sons and daughters, no matter how far the line of descent extends. (Al-Fatawa Al-Jami`ah, 2/591)
Who Is a Woman’s Mahram
Is the Father of One’s Ex-Husband a Mahram?
Is the husband of her husband’s daughter a Mahram for her?
Is My Husband’s Stepfather My Mahram?
The Maternal and Paternal Uncles of the Husband Are Not Mahram to the Wife
And Allah knows best.