Your grandfather's son's son could be:
cousin
3rd cousin
Most likely they would not be related to you
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 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.
They share a common Grandparent. That makes them first cousins.
your cousin
i am my own grandfather