As a linguistics nerd, I’d like to formally suggest “Nespring” as a nonbinary equivalent to niece/nephew and “Avaun” as a nonbinary equivalent to aunt/uncle
Nespring: literally meaning “the offspring of my sibling” from the old English roots “offspring” which is just ‘of spring’ and the Latin root “nepos” that both niece and nephew are derived from
Avaun: derived from the roots of both “aunt” and “uncle,” the anglo-French “aunte” and the Latin “avunculus”
bongdanceking 