Bellatrix is Draco and Nymphadora's aunt and Voldemort sired her daughter, does that make him their uncle?
Based on the Slytherin line, I would guess no.
We know that Voldemort (Tom Riddle) is a half blood, with his sire being Tom Riddle Sr, and that the Malfoy family prides themselves on being Pureblood. That would lead you to believe that if they were related, it would have to be on the Magical side, being the Slytherin/Gaunt side, through Merope. Since Marvolo said they were the last of the line, and the fact that they were suffering from being inbred, I would say that is one family tree that truly had few branches.
Just because you have a kid with someone doesn't make you their family. So no, Voldemort is not Draco's uncle.