A grandson of one's sibling; a son of one's nephew or niece. (Brother's grandson: fraternal grandnephew. Sister's grandson: sororal grandnephew.)

plural of grandnephew

A great-grandson of one's sibling, a grandson of one's nephew or niece. (Brother's great-grandson: fraternal great-grandnephew. Sister's great-grandson: sororal great-grandnephew.)