Nibling is a very useful neologism, which offers a neutral alternative to niece or nephew. In the same way you can cut down on your word count by saying "siblings" instead of "brothers and sisters", use "niblings" when referring to the collective offspring of said siblings.
Sadly, I have to admit that the word is entirely made up, and, worse still, by somebody other than my good self. Nonetheless, it strikes me as a word which will inevitably make its way into the lexicon, as our modern age of convenience demands. | 
