After the death of my husband I married his nephew who was the husband of my stepdaughter. Is this marriage considered valid under Sharee‘ah (Islamic law)?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

There are four types of women whom a man is permanently forbidden to marry because they are a relative through marriage:

1.       The wife’s mother, paternal grandmother, maternal grandmother up to all levels. Allaah The Almighty Says (what mean): {Prohibited to you [for marriage] are … your wives' mothers,….} [Quran 4:23]

2.       The wife’s daughter, granddaughters, daughters of her granddaughters, daughters of her sons down to all levels. Allaah The Almighty Says (what mean): {Prohibited to you [for marriage] are… your step-daughters under your guardianship [born] of your wives unto whom you have gone in.} [Quran 4:23]

3.       The wife of one’s son or grandson down to all levels. Allaah The Almighty Says (what mean): {And [also prohibited are] the wives of your sons who are from your [own] loins, …} [Quran 4:23] 

4.       The wife of one’s father or grandfather up to all levels. Allaah The Almighty Says (what mean): {And do not marry those [women] whom your fathers married, …} [Quran 4:22]

Having stated this, you and your husband’s nephew are not considered relatives through marriage and consequently, you are not prohibited from marrying him and your marriage is valid, In shaa’ Allaah (Allaah willing). Even if your husband marries you as well as your ex-husband’s daughter at the same time, this is permissible as long as she is not your own daughter. 

Allaah Knows best.