If you’re looking for a gender neutral term for the kid, we use Nibling for my sister’s kid. Comes from the N in neice and nephew, and the word sibling, and the fact that nibs are tiny and cute little pieces of things.
Oh I’m devastated that I won’t get to meet my niece or nephew, and that they’ll have to grow up not knowing part of their family just like I was denied access to part of mine for the first 15 years of my life.
But I’m happy for my sister. I know how much she’s wanted (and struggled) to have a baby, and I know it’ll have a good stable home with two smart, loving parents