Board Thread:General Discussion/@comment-27064008-20160403041313/@comment-25348898-20160414124041

Cheeseskates wrote: Pay close attention to the words. "Our" cousin. Mad Dummy, or the standard Dummy, are that cousin since they are the only training dummies in the game. Since it is "OUR" cousin, it implies it is not only Mettaton's cousin but also Napstablook's.
 * Dearest Diary: Our cousin left the farm to become a training dummy. That leaves just Blooky and I. Blooky asked me if I was going to try to become corporeal, too. They sounded so... resigned... come on, Blooky. You know I'd never leave you behind. And besides... I'd never find the kind of body I'm looking for, anyway.

Proof of it is that Mettaton was once a ghost, according to that diary entry, and the entire list of entries as well. Check here for proof. Yeah, Mettaton's house was the pink one.

Hope that helps :) Well if it's one of the Dummies, then it could either be the dummy outside of Undyne's house, though I've seen some people think that dummy is Mad Dummy, which I'll check out later, or it could be Mad Dummy or the Ruins Dummy which extend to each other since Mad Dummy refers to the Ruins Dummy as his/her cousin. So they're both Nabstablook and Mettaton's cousins.