...for nieces and nephews collectively. Mothers and fathers = parents. Brothers and sisters = siblings. Grandma and Grandpa = grandparents. Come to think of it, aunts and uncles need a word, too. Why the rest of the collective extended family is cheated out of brevity and ease, I don't know. Other languages probably have such words, but I took Latin in high school and it doesn't get me too far now that I've taken my fair share of SATs.
Let's think of a word and start using it in everyday conversation, k?