Is it permissible to marry the daughter of a woman who was nursed by my mother twice, knowing that the nursing occurred with my older sister before I was born?

If a woman breastfeeds a child, that child becomes her son through suckling and a sibling to her children. Consequently, any woman who breastfeeds from your mother becomes your sister through suckling, and you become an uncle to her children. It is then forbidden for you to marry her daughter, based on the Prophet’s (peace be upon him) saying: "What is forbidden by lineage is also forbidden by suckling."

However, the prohibition is only established by five known sucklings, according to Aisha’s narration: "Among what was revealed in the Qur’an were ten known sucklings that establish prohibition, then they were abrogated by five known sucklings." A single suckling means latching onto the breast, sucking from it, and then releasing it. This is not interrupted by breathing or brief pauses.

Therefore, if the woman has completed five full sucklings from your mother in this sense, it is forbidden for you to marry her daughter. But if she suckled less than five times, the prohibition is not established, and marriage is permissible. If there is doubt about the number of sucklings, the prohibition is not established, as the default is its absence.