Well since niece and nephew are so far removed from sister and brother, I just didnt feel like sibling was relevant.
Perhaps an amalgamation of the words is better Niesphew? Sounds like a sneeze.
Alternatively it can make use of the vowel "a" and be first in line. Nabble and nabling are both acceptable.
I would however still argue against having it be like "sibling" since my sister is a sibling like how my nabble would be a nabbling