The Sister's Husband is not a Mahram
Question :
Is it permissible for my sister to veil herself from her cousin, who is a Nasih for us, that is, that he will marry his daughter to my brother, bearing in mind that the marriage has not yet taken place. Advise us.
Answer:
Your sister must veil herself from her uncle's son, who is not her Mahram even if he is a Nasih, and even if he married his daughter to her brother. This is because the sister's husband is unrelated, as is the brother's father-in-law and others like them.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 5 Page 134