Does the ruling on fosterage extend to the siblings of my foster brother, thereby prohibiting marriage to their daughters?

There is no objection to your marrying the daughters of your other paternal uncles who were not suckled by your mother. As for your paternal uncle who was suckled by your mother, he is your brother through suckling, and his daughters are the daughters of a brother through suckling, and you are their paternal uncle through suckling. This is provided that he was suckled five full sucklings.