Chances are they are supposed to be related. Remember the family tree that the Blacks had and how many other wizarding families were part of it including a Potter and a Weasley. It only makes sense that with Britain's small wizard population that that pureblood family would mostly be related by marriage at least if not by blood. We don't know if the Lovegoods were purebloods but both Luna's mother and father were wizards so it stands a good chance that her father came from a wizarding family.