Source: Fatawa Aman Puri by Sheikh Ghulam Mustafa Zaheer Aman Puri
Question:
A husband has converted from Christianity to Islam, but his wife remains Christian and has not converted. Can the new Muslim husband marry his Christian wife's Muslim sister?
Answer:
If a Christian man converts to Islam, his marriage to a Christian woman remains valid; therefore, he cannot marry his Christian wife's Muslim sister, as it is not permissible to have two sisters in one marriage. The command of Allah is:
﴿وَأَنْ تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ﴾
Reference: (An-Nisa: 23)
"And you are forbidden to marry two sisters together."
