Is the infant brother of the paternal uncle’s wife considered a foster brother to the paternal uncle’s daughter who nursed from the same wife, and what is the ruling concerning the siblings of this brother?

Your older brother and your young female cousin are siblings through رضاعة (nursing) if they both suckled from the same breast and consumed five or more full nursings of your uncle's wife's milk. As for your other siblings, the nursing has no effect on them. Based on this, your female cousin is a non-mahram to you all, and it is permissible for any of you to marry her.