Board Thread:Wiki Discussion/@comment-9307814-20170501102858/@comment-27701762-20170501162820

The dummies are "possible cousins" because there is specific evidence in the game that Napstablook's and Mettaton's mutual cousin left to inhabit a dummy. We don't know exactly which one.

Meanwhile, the familial relationship between Sans, Papyrus, and Gaster has no evidence beyond speculation and the likelihood that Gaster's name is also based on a font.