Is it permissible for Fatima and Abdullah to marry if Zainab's grandmother breastfed them both when they were under two years old?

If Abdullah is Fatimah's paternal uncle by lineage, then he is her mahram.

However, if it is meant that Hind has a son named Abdullah, and Fatimah and Abdullah, or one of them, suckled from Zainab's grandmother five known sucklings, then neither of them is permissible for the other. This is because if they both suckled, they are foster siblings. If only one of them suckled, the other would be the son of a sister or the daughter of a brother. The Prophet, peace and blessings be upon him, said: "Fosterage prohibits what lineage prohibits." And among those prohibited by lineage are the son of a sister and the daughter of a sister, no matter how far down the lineage.