Your children and your grandniece first cousins, once removed, but in conversation they would call each other by their first names.
Great grandniece
You are your grandniece's greataunt or greatuncle. Your grandneice may call you her greataunt, or if close to you, may call you simply aunt (or uncle).
Your husband's grandniece is your grandniece-in-law.
Your grandniece's daughter is your great-grandniece.
Your sister's children's children are her grandchildren.Your children and your sister's grand children are first cousins, once removed.Your grandchildren and your sister's grandchildren are second cousins.
Your grandchildren and your grandniece are second cousins to each other..
Your grandniece and your daughter are first cousins, once removed, to each other.
Grandniece
grandniece
Your grandniece is the granddaughter of your brother or sister, and the daughter of your niece or nephew. Your grandnephew is the grandson of your brother or sister, and the son of your niece or nephew.
Your brothers granddaughter is your grandniece.
She is his great grandniece.