What is the ruling regarding a man who marries his sister's granddaughter?

This marriage is invalid because the daughter of one's sister—no matter how many generations removed—is among the women forbidden by the text of the Quran. Separation between them must occur immediately. If the marriage took place due to error or oversight, then the man is excused. However, if it was intentional, he must promptly initiate sincere repentance, seek forgiveness, and increase good deeds.