Your great-niece.
Niece/Nephew
Your great-niece.
Great grandniece
Your nephew?
Your niece and nephew.
you can call him your nephew if you are mom's daughter's sibling or if you will be the mom you can call him your grandson.
Your daughter is your sister's niece. Your daughter's son is her great nephew.
He would be your grand-nephew.
your nephew or neice
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
The daughter of your brother is your niece.
Your sister's daughter, to you, is your neice. (You are her aunt or uncle). If the daughter were actually a son, he would be you nephew.