Draco has a tapestry at his house that shows his family tree but I don't think it goes back a thousand years. And Hydra's family has one too. They'd surely have mentioned it if it traced back to Slytherin but then Slytherin wouldn't have an HEIR he'd have a whole pack of heirs because all the old families are related. Ron's even related to Draco through a cousin, I know that must be true Ron because I heard your mum inherited something from Draco's great-aunt who died a few weeks ago. And there are five Weasleys at the school right now so that would be five more heirs for party.

Really, if anyone's related to Slytherin then practically everyone is, if it's as simple as being a blood descendant.