Nephling

My dear neph-ling: I congratulate you on the attainment of your degree as “Master of Arts.” […] Your affectionate uncle, Cinna Beverley.

One of my correspondents uses the idiolect form "nephlings" to refer collectively to her niece and nephew, but points out that even this derives from the masculine form.

June 19 is six days before mine. And 4 days after the nephling’s.

The nephling, now 4-1/2 is a second child also, and didn't speak more than a word or two until he was close to 3-1/2.

Tell us about the new nephling. […] She’s cute.

A bachelor of our acquaintance describes his brother's children as his “nephlings.” This term is not only gender neutral, unlike niece or nephew, but implies a closer relationship with its connection to the word sibling.