Is it permissible for him to marry the daughter of his wife’s sister after she dies
Question

                There is a man who married the daughter of his wife’s sister after she died. What is the ruling on that?.
Praise be to Allah.
    Firstly: 

    It is permissible for a man to marry the daughter of his 
    wife’s sister after his wife dies, because what is haraam is to be married 
    to the girl and her maternal or paternal aunt at the same time. But as the 
    maternal aunt has died, what is forbidden no longer applies, which is being 
    married to her and her maternal aunt at the same time. So there is nothing 
    wrong with his marrying her in that case. 

    It says in Tanqeeh al-Fataawa al-Haamidiyyah (1/111) – 
    a book of Hanafi fiqh –: 

    He was asked about a man whose wife with whom he had 
    consummated the marriage died, and she had a sister; is it permissible for 
    him to marry her sister one day after she died? 

    Answer: Yes, as it says in al-Khulaasah narrating from 
    the book al-Asl by Imam Muhammad; and as it says in Mabsoot Sadr 
    al-Islam, as quoted from him by al-Quhustaani; and as it says in 
    al-Muheet by Imam as-Sarkhasi… End quote. 

    Shaykh Ibn Baaz (may Allah have mercy on him) was asked:

    Is it permissible for a man to marry the daughter of his 
    wife’s brother? 

    He replied: It is not permissible for a man to marry the 
    daughter of his wife’s brother if the girl’s paternal aunt is still married 
    to him, just as it is not permissible either for him to marry the daughter 
    of his wife’s sister, if her maternal aunt is still married to him, because 
    the Prophet (blessings and peace of Allah be upon him) forbade a man to be 
    married to a woman and her paternal aunt at the same time, or to a woman and 
    her maternal aunt at the same time. 

    The scholars (may Allah have mercy on them) were unanimously 
    agreed that that is forbidden because of this saheeh hadeeth. But if the 
    paternal or maternal aunt dies or he has divorced her and the ‘iddah has 
    ended, then there is nothing wrong with him marrying the daughter of her 
    brother or the daughter of her sister, because in this case he will not be 
    married to them both at the same time.

    End quote from Majmoo‘ Fataawa Ibn Baaz (21/8) 

    Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: 

    A man married a woman and she bore him children. Now he wants 
    to marry the daughter of his wife’s sister; is that permissible? 

    He replied:

    The wife is the maternal aunt of that sister’s daughter, and 
    the Prophet (blessings and peace of Allah be upon him) said: “(A man) should 
    not be married to a woman and her paternal aunt, or to a woman and her 
    maternal aunt, at the same time.” So it is not permissible for him to marry 
    the daughter of his wife’s sister so long as he is still married to his 
    wife. But if he divorces her or she dies, and he marries her sister’s 
    daughter, there is nothing wrong with that. 

    End quote from Fataawa Noor ‘ala ad-Darb (19/2)  

    And Allah knows best.