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.
Great-great Aunt
Second Cousin
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your son's children are your grandchildren.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your grandfather's son's son could be:you (if you are a boy)your brotheryour first cousin
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Nieces are the daughters of your brothers and sisters. Nephews are the sons of your brothers and sisters.
They share a common Grandparent. That makes them first cousins.
Great-niece.
The nephews and nieces have equal standing under the law. However, if one wishes to speak of royal precedent, then the deceased's eldest sister's eldest son, and if she had no son, then the eldest son of the oldest sister who did have sons.
your cousin