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 sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your great nephew is your kids' first cousin once removed.
You are their aunt or uncle and they are your niece or nephew.
Your uncle's grandchildren are your first cousins, once removed. They and your grand children are your second cousins, once removed
Your son's children are your grandchildren.
They share a common Grandparent. That makes them first cousins.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
Your nephew's son is your great nephew.
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.
First cousin, once removed. Your parent on that side would be a first cousin to the nephew. If the nephew has children, they will be your second cousins. (Of course, your great uncle's nephew could also be your father).
A great great grandchild and a great great nephew of the same person are third cousins, once removed.