Subling

Coined by Stephen Kerr of Wagga Wagga, NSW, Australia.
'Subling' is the collective noun for "nephews and nieces" and can be directly substituted.
It is derived from an amalgamation the words 'sub' meaning under or below and 'sibling' the collective noun for "brothers and sisters"

Original: "I am keen to see my nephews and nieces these Christmas holidays."
Replaced: "I am keen to see my sublings these Christmas holidays."