Are the daughters of a wife with whom consummation has not occurred permissible to marry?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
The daughters of a wife with whom consummation has not occurred (i.e., stepdaughters/Raba'ib) are lawful to marry because of the verse: "But if you have not consummated [the marriages with them] - then there is no blame upon you" (Quran 4:23). This prohibition is mentioned to clarify the ruling, as the mother's affinity is established upon contract, but here it is not.
Supporting text
The daughter of a stepdaughter is forbidden because the stepdaughter herself becomes a stepdaughter (Rabi'ba), and the rationale for prohibiting the Rabi'ba—difficulty in avoiding looking at or being in seclusion with her as she is in one's household—applies to her daughter even if remote. The wife married by the father or son is forbidden by the marriage contract, but this rationale does not apply to her daughter.