So if Harry is descended from Ignotus and Voltemort from Peverrels, does that make them long lost cousins?
Yes, they are related.
(Tom Marvolo Riddle is Voldemort as you remember :) )
While they are "related" throgh Peverell brothers, as per @nicael's answer, they aren't "related" any closer:
Question: Is Voldemort some sort of relative of Harry's? Possibly his mother's brother?
J.K. Rowling responds: I'm laughing...that would be a bit Star Wars, wouldn't it?
(source: "About the Books: transcript of J.K. Rowling's live interview on Scholastic.com," Scholastic.com, 16 October 2000)