My husband wants to marry my niece. I am not opposed to it, because he is a good man and she we both will benefit greatly. I am just having a hard time finding if it's legal.
According to Surat An Nisa'
Prohibited to you (For marriage) are:- Your mothers, daughters, sisters; father's sisters, Mother's sisters; brother's daughters, sister's daughters; foster-mothers (Who gave you suck), foster-sisters; your wives' mothers; your step-daughters under your guardianship, born of your wives to whom ye have gone in,- no prohibition if ye have not gone in;- (Those who have been) wives of your sons proceeding from your loins; and two sisters in wedlock at one and the same time, except for what is past; for Allah is Oft-forgiving, Most Merciful;- (4:23)
The Sister of the wife and her descendants are woman who are haram to marry simultaneously to the actual wife. If the man divorced his wife or she died he can marry either her sister or her niece. One could indirectly conclude this from the above verse or better rely on the sunnah:
"One cannot be married to a woman and her paternal aunt, or a woman and her maternal aunt at the same time."
(Sahih al-Bukhari, Sahih Muslim, Muwatta' Malik, Sunan an-Nasa-i, Sunan ibn Majah, Sunan abi Dawod and Jami' at-Tirmdihi with different versions)
For your niece you would be her maternal aunt!
So according to this Fatwa as long as the wife is alive or still married to his current wife the husband is prohibited to marry her niece. As the haram (not mahram as they are no real mahrams) status stays as long as the wife is alive or still in his hands.
This is the Sunni point of view i found a shia Fatwa which allow it only if the wife herself agrees to that marriage!
No, he can't. While you two are married, your husband is an uncle to your niece, and marrying one's uncle is not allowed by consensus. Following links may help further in understanding the issues:
Yes, it is halal.
The Ibn Abbas quote is true, but you should look at Tafseer Ibn Abbas. It clearly states the principle of precedent. If a man marries a woman; then there is no issue in him marrying her niece too. However, if he marries the niece first and consummates the marriage, then he cannot marry the aunt until he either divorces the niece or the niece dies.