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 grandniece's daughter is your great grandniece.
Great Aunt "insert name." and or Great Uncle "insert name."
Your niece's granddaughter is your great great niece
Your niece can cll you aunt or uncle, in whatever language, as applies. Or whatever else you are both comfortable with, such as your name or a nickname.
Your children and your grandniece first cousins, once removed, but in conversation they would call each other by their first names.
Great grandniece
Your husband's grandniece is your grandniece-in-law.
Your grandniece's daughter is your great-grandniece.
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.
She is his great grandniece.
The plural of grand niece is grand nieces.
your 2nd niece...!