Prince Blueblood is quoted in MLP season 1 as Princess Celestia's nephew, but I have to assume he is also Princess Luna's to the nth degree great grandson (separated by many generations). Luna married a male unicorn, and their descendants were considered part of Equestrian royalty. On the other hand, Blueblood could actually be Luna's son. That would imply Blueblood might be a semi-alicorn, meaning he ages almost as slowly as Celestia and Luna, but lacks wings.