On the Sports Personality of the Year show tonight, the winner was trying to find a collective term to describe the child of his sister. So he had to turn the sentence around and describe him self as someone who had 'just become an uncle' which covers the relationship irrespective of the gender of the child (nephew or niece). Yes, he admitted eventually that his sister had given birth to a little girl, and that therefore she was his niece. And it would have still been his neice even if his sister-in-law had given birth. And, given the way some folks carry on these days, the actual wearing of a ring or ownership of a licence is immaterial.
But that's my point.
There is no accepted word for such offspring.
Consider the following options.
One's first Grandchild/Granddaughter/Grandson
One's first child/daughter/son
One's first nephew/niece BUT there isn't a generic term.
That's why I have been campaigning (okay, fairly quietly) for Sibskid as that term. My sibling's kid, geddit? It's friendly, easy to use, applies equally to boy or girl born of sister or of the seed of brother without argument, and is easy to remember (which is why it beats SibsSpring, which is harder to say, or NéeBoth, which is arguably more etymologically excusable).
Let me know what you think, unless you disagree.