Your nephew and grand nephew may be parent and child. In not, then they are First Cousins, Once Removed.
Grand-Nephew
Your grandson and your great nephew are second cousins to each other.
Your great nephew and your granddaughter are second cousins to each other.
Your nephew and your daughter are first cousins to each other.Your nephew's son and your daughter are first cousins, once removed, to each other.
Your nephew and your daughter are first cousins to each other. Your nephew's son and your daughter are first cousins, once removed. Your nephew's son and your daughter's child would be second cousins to each other.
Your son and your nephew are First Cousins. The children of your nephew and the children of your son are Second Cousins to each other.
Your son and your grand niece are first cousins, once removed. Explanation: Find the common ancestor, in this case, your parents. Your parents have two children, you and your sister or brother. Your sister or brother has a son or daughter. This child would be your nephew or niece and your son's 1st cousin. A daughter of your nephew or niece is your grand-niece, thus your son's 1st cousin once removed. Note: Your grand-niece and your son's child would be 2nd cousins. Hierarchy: 1. Any people 2. sons and daughters are siblings to each other 3. grandchildren are first cousins to each other 4. great-grandchildren are second cousins to each other
Your granddaughter and grand niece are second cousins to each other.
As far as I can make out, Your daughter is your fathers (common ancestor) grandchild, and your grand nephew (not great nephew) is your fathers great grandson. According to the chart at http://freepages.genealogy.rootsweb.ancestry.com/~gentutor/chart.html this would make your grand nephew, your daughters First Cousin once removed
Your child and your great nephew's parents would be first cousins. Their children would then be second cousins. And if they have children, they will be third cousins.
You are the grand uncle to any children your nephew or niece have.They would be your grand nephew or grand niece.If the children of your nephew/niece have children of their own, you'd be a great uncle too.
Your daughter and your nephew are first cousins to each other. Your daughter and your nephew's son are first cousins, once removed, to each other.