You would be second cousins.
Your great great niece is the granddaughter of your niece or nephew. Your daughter and your niece or nephew are first cousins. Your daughter and your great great niece are first cousins, twice removed.
Your grandfather's great great niece is your second cousin, once removed
Your husband is great uncle to your niece's son (your great nephew).
Great-niece.
great nephew
You are the great-great-aunt/uncle to your niece's granddaughter. This means that your niece's granddaughter is your great-great-niece. The term "great-great" signifies that there are two generations between you and your niece's granddaughter.
They are cousins.
There probably isn't a relationship. You do not share a common ancestor.
Your brother's daughter is your niece. Her daughter is your great niece.
Your niece's son's daughter is your great grand niece. Your niece's son is your grand nephew or your great nephew, whichever term you prefer.
Great niece
He or she is your great-niece/nephew