The incestious offspring of your grandmother and your brother, or your grandfather and your sister. Being the brother of your mother, and therefore your uncle, and the son of your sister, and therefore your nephew. He is your respectable Uncle Nephew, and he is quite a debauchery of gene mixing.
Good Ol' Leonard Bobby Ferguson was sitting on the top of Mount Rushmore one day when he saw his Grandmother wearing a very revealing outfit.
See, him and his Grandmother were only about twenty years apart due to the family tradition of giving birth at the age of ten. Leonard, who I'll refer to from now on as Leo, felt the old feelings rising in him again and brought his family to a new low by commiting an act of incest with his own grandmother, which she enjoyed just as much as he did...for fuck's sake.
Anyway, eight (not nine) months later their offspring was born. Leo's brother Siegfried reluctantly called this new family member his "Uncle Nephew."