Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
cousin
Second Cousin
3rd cousin
Your aunt's grandchildren would be your first cousins once removed, as the your aunt's children are your first cousins.
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Your niece's son's son is her grandson. He is your great great nephew and you are his great great aunt or great great uncle, depending on your gender.
Your grandmother is first cousins with cousin. Your parent would be the second cousin of the cousin's son. That would make you second cousins once removed.
Great-niece.
Your father's nephews and nieces are your first cousins.
Nephews and nieces
your 2nd niece...!