Is it permissible for a man to marry the sister of a woman he has divorced or separated from?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
If a man separates from a woman, he is not permitted to marry her sister until the first woman has been freed from any potential pregnancy (istibra'), and her non-pregnancy is confirmed. If the woman is pregnant by him, her sister is not permissible for him until she delivers her child.
Supporting text
The prohibition against marrying the sister while the first wife is pregnant by him is because this action is likened to combining the semen into the wombs of two sisters, similar to the ruling on marrying a woman during her waiting period ('iddah) for her sister.