Wiktionary
nestmate
n. One with which a nest is shared, such as a baby bird and its siblings or figuratively a child with children it is raised with.
Usage examples of "nestmate".
She remembered now the sight of her nestmates pressing chewed food on one another.
Ordinary crewfolk of the several ships go more freely about on their leaves, but may not speak to anyone, no, not nestmates or clones or even each other, of what they have done and seen.
Then, at the beginning of fall, the vampires of Dallas asked the vampires of Shreveport if they could borrow me for a little investigation into a missing nestmate of theirs (Living Dead in Dallas).