That would be your niece or nephew. Niece in Afrikaans is niggie, and nephew is neef
kind
soogdier
aarbei
gelukkige kind
Arkmark
Wildtuin
David
We call it "valskermspring"
Niece if it's a female, and nephew if it's a male.
Afrikaans
Flock of chickens
Your sister's daughter is your niece. You are her aunt if you are female, or her uncle if you are male.