If your a boy they would call you great uncle, if your a girl they would call you a great aunt.
Your niece/nephew should call you 'Aunt' (female) or 'Uncle' (male).
Your niece's children call you and your husband their great aunt and great uncle.
Your relationship to them is grandaunt. What they call you is up to you and your niece to decide, maybe aunty -- grandaunt is a bit of a mouthful.
You are a great-aunt.
Your brother's daughters are your nieces
Your brother's daughters are your nieces
Nieces.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
The daughters of your sister are your nieces.
Your niece's daughter is your grand niece, or, as some would say, your great niece.
Nieces are the daughters of your brothers and sisters. Nephews are the sons of your brothers and sisters.
I can give you several sentences.My nieces are coming for a visit.Are those girls your nieces?Nieces are the daughters of your brother or sister.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
If you have two nieces who are not sisters but are each the daughters of your siblings, the two nieces are first cousins. If they each have a daughter, those daughters are second cousins to each other.
You call them your nieces, or by their names.
My niece's daughter is my great niece.