There is no difference between "grandniece" and great niece." Some people prefer to use "grand" but others prefer to use "great" when talking about the granddaughter of one of their siblings.
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.
No. A great grand child is the child of your grandchild. The child of your grand niece is your grand niece or grand nephew, depending on the child's sex.
It's your cousin. Lets say: Grandma Wilma has a brother Bill. Bill is your great Uncle. His daughter Joan is your parent's First Cousin and your First Cousin, once removed.
Your grandniece's daughter is your great-grandniece.
She is his great grandniece.
Great grandniece or great grandnephew
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).
Great grandniece
your 2nd niece...!
There is no difference between "grandniece" and great niece." Some people prefer to use "grand" but others prefer to use "great" when talking about the granddaughter of one of their siblings.
Your husband's grandniece is your grandniece-in-law.
She would be your Great/ Grand Niece. Seeing as you are her Great Aunt or Uncle.
You are her grand uncle (or great aunt) and she is your grand niece.
Your grandchildren and your grandniece are second cousins to each other..
Your granddaughter and your grandniece are second cousins or third cousins to each other