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