What is the ruling on a man marrying a woman whom his sister breastfed?

Question:
I want to marry my cousin’s daughter, but I learned that this girl was breastfed by my older sister after my sister’s son had grown up. We do not know how much she breastfed her. The father of the girl I want to marry is not pleased with this breastfeeding because my sister came and breastfed her without prior permission. So, should I marry this girl or not?

Answer:
If your sister breastfed her completely—five times or more—then this girl is your sister, and she is not permissible for you. However, if the breastfeeding is not specified, not known, and it is not known whether it was one, two, or three times, then it does not make her forbidden to you. This is because the breastfeeding that makes a woman forbidden must be five known feedings within the first two years. This is based on what is established from him (peace and blessings be upon him) that he said: , and he said to Sahlah bint Sahil: . Aisha (may Allah be pleased with her) said: “It was among what was revealed in the Qur’an: ‘Ten known feedings make a woman forbidden.’ Then this was abrogated by five known feedings. The Prophet (peace and blessings be upon him) passed away while the matter was still as such.” In another version: “By five known feedings,” and the Prophet (peace and blessings be upon him) passed away while it was still being recited in the Qur’an.
This indicates that the breastfeeding that results in prohibition must be five known feedings while the child is within the first two years. If your sister breastfed her five times within the first two years, then you are her uncle, and she is not permissible for you. However, if your sister does not know whether she breastfed her once, twice, or three times, then she is not forbidden to you on that account. But leaving her alone is in the category of avoiding doubt, and in the category of , which is better.