Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, states: “It is permissible for you as a muslim who adhere to Islamic rulings to marry the daughter of your maternal aunt if the breastfeeding took place twice, because breastfeeding (rada`ah) only makes the woman (and her daughters) the mahrams of the child who nursed if it takes place five times. [A mahram is a relative whom one is forbidden to marry and with whom the rulings of hijab or covering do not apply]. The evidence for that is the hadith narrated by Muslim from `A’ishah who said: “One of the (rulings) that was revealed in the Qur’an was that ten known sessions of breastfeeding make the child a mahram, then that was abrogated and replaced with five.”
An-Nawawi (may Allah have mercy on him) said: The scholars differed concerning the number of sessions of breastfeeding for which the ruling on breastfeeding (rada`ah) applies. `A’ishah and ash-Shafi`i and his companions said that there is no proof for any number less than five. The majority of scholars said that the ruling applies if breastfeeding occurs once. This was narrated by Ibn al-Mundhir from `Ali, Ibn Mas`ud, Ibn `Umar, Ibn `Abbas, `Ata’, Tawus, Ibn al-Musayyib, al-Hasan, Makhul, al-Zuhri, Qatadah, Hammad, Malik, al-Awza`i, al-Thawri and Abu Hanifah (may Allah be pleased with them). Abu Thawr, Abu `Ubayd, Ibn al-Mundhir and Dawud said: The ruling applies in the case of three sessions of breastfeeding, not less than that. Ash-Shafa`i and those who agreed with him followed the hadith of `A’ishah which mentioned five known session of breastfeeding.
Sheikh Ibn Baz was asked whether breastfeeding from a woman three times makes her a mahram.
He replied: this breastfeeding three times does not make her a mahram through breastfeeding. The ruling on becoming a mahram through breastfeeding only applies if breastfeeding occurs five times or more. Then he quoted the hadith of `A’ishah as evidence. (Source: Fatawa Islamiyyah, 3/326)
Sheikh Ibn `Uthaymeen said:
One session of breastfeeding does not have any effect, rather it must be five sessions of breastfeeding that occur before the child is weaned and before he reaches the age of two. A person does not become the woman’s (foster) child if he breastfeeds once or twice or three or four times. It must also be five known sessions of breastfeeding; if there is some uncertainty as to whether he breastfed four or five times, the principle is that it was four, because every time we are uncertain about numbers, we take the lower number. Based on this, if a woman says, I breastfed this child but I do not know if it was once or twice, or three or four or five times, we say that this child is not her (foster) child, because it has to be five known sessions of breastfeeding without a doubt. (Source: Al-Fatawa al-Jami`ah lil-Mar’ah al-Muslimah, 2/768)”