You can call them their real name...or...niece
Your brothers child is either a nephew or a niece to you.
Your sister's daughter is your niece. You are her aunt if you are female, or her uncle if you are male.
That would be your niece or nephew. Niece in Afrikaans is niggie, and nephew is neef
Your niece or nephew
Your child would call your sister her aunt.Your sister would call your child her niece or nephew depending on gender.Your sister's children would call your child their cousin.
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
A girl would be your niece and a boy your nephew.
If it's a boy: Nephew If it's a girl: Niece
Your niece's child is your great niece if she is a girl, or your great nephew if he is a boy.
Your niece and your child are first cousins. Your niece's child and your child are first cousins, once removed. The children of your niece's child and of your child are second cousins to each other.
You are Great Aunt to your niece's child. The child is your great niece or great nephew.
Your child and your niece are first cousins to each other. Your child and your niece's child are first cousins, once removed.