Prohibition of Suckling Sisters

Question:
This is a letter we received from the sender A.K.A, from Iraq, from Nineveh. He says in his letter: We are brothers with a sister of ours; one is twenty-five years old, the second is twenty-three, and the sister is twenty years old. He says: I also have a cousin who is also twenty years old, because my mother breastfed my sister and my cousin. He asks: Is my cousin forbidden to me if I want to marry her, or not? Please inform us, may Allah reward you with the best reward.
Answer: If your mother breastfed her five times or more, she becomes your sister, and it is not permissible for you to marry her; because Allah has forbidden through breastfeeding what He has forbidden through lineage, and Allah has explicitly stated in the Qur'an the prohibition of suckling sisters. Therefore, it is not permissible for you to marry the daughter whom your mother breastfed with complete breastfeeding; because through breastfeeding she has become your suckling sister. And Allah (Exalted and Majestic is He) said regarding the prohibited women: [Al-Nisa: 23], and the Prophet (peace be upon him) said: So if this girl was breastfed by the mother—meaning your mother—five times, whether in one sitting or in multiple sittings, or more than five times, then she becomes your sister. A breastfeed is when the child, boy or girl, holds the breast and sucks the milk... swallows the milk, then is separated from the breast, then returns to suck, then returns again until five are completed. If five are completed in one sitting, or in multiple sittings in one day or one night, or in days or nights, then this breastfed child becomes a son to the nursing woman, and a son to her husband, the owner of the milk, and a brother to all her children and the husband's children. And by this you know that this breastfed daughter of your mother is not lawful for you; rather, she is your suckling sister if it is established that your mother breastfed her five times or more. As for if the breastfeeding was little, one or two times, there is no harm; she does not become your sister; because the Prophet (peace be upon him) said: .
Likewise, if it was three or four until the five are completed; because the Sunnah came with five breastfeeds being the ones that prohibit marriage. And likewise, if there is doubt, and your mother says: "By Allah, I do not know if she breastfed her two, three, four, or five times; I have no record." If your mother is in doubt about the number, then this girl is not prohibited to you. But if you leave her off the basis of precaution... on the basis of avoiding doubtful matters, that is better; because she is a woman in whom there is suspicion, so leaving her and staying away from her is preferable. However, she does not become your sister except with five breastfeeds or more. You should not search for her or look at her face or the like; rather, she must observe modesty and the hijab until your mother is certain that she breastfed her five times or more. Yes.