What is the ruling when a man converts while married to two sisters?

Primary text

The marriage to the sister of the wife is invalidated. The evidence for this is the saying of God: {And the mothers of your wives} [Quran 4:23], which includes the mother of his wife. Furthermore, as she is the mother of his wife, she becomes permanently forbidden to him, similar to the case if he had divorced the daughter while in a state of disbelief (*shirk*). Moreover, if he had married only the daughter and then divorced her, her mother would become forbidden upon his Islam. Since he maintained the marriage to her, this precedence is stronger. The view that the contract only becomes valid upon choice is incorrect, as the contracts of non-Muslims are valid, and their rulings are established. If the marriage were isolated (to the daughter only), her marriage would be binding without the need for choice, which is why choice is delegated here. The mother is specifically singled out for nullification because her marriage becomes permanently forbidden merely by contracting the marriage to her daughter, preventing choice, whereas the daughter is not forbidden before consummation with the mother, thus the marriage remains valid in her case, unlike the case of two sisters.