Board Thread:General Discussion/@comment-28292244-20160425154019/@comment-27701762-20160425183804

So Mettaton and Napstablook are definitely cousins, since the house belongs to Mettaton, and is described as "Napstablook's cousin's house."

It's speculated that either the dummy from the beginning of the game or the Mad Dummy are also their cousin. Given that the dummy and Mad Dummy are cousins, it is possible that all four of them could be related in some way to one another. However, we aren't entirely certain which is which since we have two ghosts who have inhabited dummies from the game but one being described in the diaries. So the question becomes "which one is the diary referring to?"