Is it permissible to marry the sister of a girl with whom I was breastfed full and separate times by her mother?

Scholars differed regarding the amount of breastfeeding that establishes prohibition. The Maliki and Hanafi schools held that a small or large amount is the same, provided it reaches the child's stomach within the two years of breastfeeding. They cite the verse: "and your mothers who suckled you," and the general Hadith: "What is prohibited by lineage is prohibited by breastfeeding." In contrast, the Shafi'i and Hanbali schools maintained that only five or more breastfeedings establish prohibition, and this is the preferred and authoritative opinion. Based on this latter opinion, there is no harm in marrying one's cousins if the number of breastfeedings did not reach five, though it is advised to refrain from this out of piety.