If a woman breastfeeds a child and fulfils the conditions that will be mentioned in Ruling 2492, the father of that child cannot marry the woman’s biological daughters. In the event that one of them is presently his wife, the marriage contract becomes invalid (bāṭil). However, it is permitted (jāʾiz) for him to marry her nursing daughters, although the recommended precaution (al‑iḥtiyāṭ al‑mustaḥabb) is that he should not marry them.
Furthermore, he cannot, based on obligatory precaution (al‑iḥtiyāṭ al‑wājib), marry the biological and nursing daughters of the nursing father. In the event that one of them is presently his wife, then based on obligatory precaution, the marriage contract becomes invalid.