He breastfed from his maternal aunt twice; can he marry her daughter?
Question

I proposed marriage to the daughter of my maternal aunt, but when the time for marriage drew close my aunt told me that she had breastfed me twice when I was little, but I had not drunk my fill on those occasions. Is it permissible for me to marry her?.
Praise be to Allah.It is permissible for you to marry the daughter of your maternal aunt in this situation, because breastfeeding (radaa’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 - Translator]. The evidence for that is the hadeeth narrated by Muslim (1452) from ‘Aa’ishah who said: “One of the (rulings) that was revealed in the Quran was that ten known breastfeedings make the child a mahram, then that was abrogated and replaced with five.”    
Al-Nawawi (may Allah have mercy on him) said: 
The scholars differed concerning the number of breastfeedings for which the ruling on breastfeeding (radaa’ah) applies. ‘Aa’ishah and al-Shaafa’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’ood, Ibn ‘Umar, Ibn ‘Abbaas, ‘Ata’, Tawoos, Ibn al-Musayyib, al-Hasan, Makhool, al-Zuhri, Qataadah, Hammaad, Maalik, al-Awzaa’i, al-Thawri and Abu Haneefah – may Allah be pleased with them. Abu Thawr, Abu ‘Ubayd, Ibn al-Mundhir and Dawood said: The ruling applies in the case of three breastfeedings, not less than that. Al-Shaafa’i and those who agreed with him followed the hadeeth of ‘Aa’ishah which mentioned five known breastfeedings. 
For the definition of breastfeeding which makes the child a mahram, see question no. 804. 
Shaykh Ibn Baaz 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 hadeeth of ‘Aa’ishah as evidence. 
Fatawa Islamiyyah, 3/326. 
Shaykh Ibn ‘Uthaymeen said: 
One breastfeeding does not have any effect, rather it must be five breastfeedings 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 breastfeedings; 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 breastfeedings without a doubt. 
Al-Fatawa al-Jaami’ah li’l-Mar’ah al-Muslimah, 2/768.