She would be your first cousin, once removed.
Your uncle's granddaughter would be your second cousin. Since the child's parent is your cousin, this makes the new generation your second cousin.
Your niece's grandchild is your your great great niece or great great nephew, and you are either their great great aunt or great great uncle.
I would say that she would be your aunt.
Great Aunt
The children of your grand nephew (or great nephew) are your great great nephews and great great nieces (depending on whether they are girls or boys).
My twin nieces have their birthday soon.One of my nieces has fallen asleep in front of the television.
The correct plural possessive of nieces is nieces'.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
The plural of niece is nieces. The possessive form for nieces is nieces'.
Biologically, the grandchild is a step-grandchild to the grandparent that had no children. In matters of the heart, however, the grandchild is whatever the grandparent feels it is.
Your great nieces or newfews
Grand niece.
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.
The plural of grandchild is grandchildren.
(The word nieces' is a plural possessive noun, referring to more than one niece.) The elderly aunt visited her nieces' classroom. The couple attended both of their nieces' weddings.
The grandchild of your grandmother's first cousin is your third cousin. The grandchild of your grandmother's second cousin is your fourth cousin. The grandchild of your grandmother's third cousin is your fifth cousin.