Perhaps I don’t think about cousins enough to have considered that. To me “sibling” refers to my brothers and sisters, and therefore extends naturally to “their kids” more than to other family members on the same generation. The old English word that sibling was revived from meant “kinfolk” and would have included all family whether brothers, nieces, cousins or aunts.

If I talk about “my nildren” it’s maybe a bit too possessive, and “nids” Is gross, but I’d be open to other suggestions! Niblings is defintely kinda silly, which was part of the charm when they little anklebiters.