Would a girl adopted by a childless couple and then breastfed by the paternal uncle's wife by suckling, consider all members of the paternal family by suckling to be her cousins?

Adoption that attributes the adopted child to the lineage of the adopter is impermissible, due to the Almighty's saying: "Call them by [the names of] their fathers; it is more just in the sight of Allah."

As for the fostering sister suckling the fostered girl, if there are five known sucklings within the first two years of the child's life, then the girl becomes a mahram to the fostering sister, and is considered like her sister's daughter. The same applies to all the brothers and sisters of the nursing woman; the girl becomes the daughter of their sister who nursed her.

However, the sons of her paternal uncles and aunts, and the sons of her maternal uncles and aunts, are not mahrams, whether by lineage or by suckling, due to the Prophet's (peace be upon him) saying: "Suckling makes unlawful what birth makes unlawful."