Board Thread:General Discussion/@comment-28431168-20160617130858/@comment-33883848-20190701133030

I think that W.D.Gaster is Sans and Papyrus's another brother as hinted by Toby Fox in his official undertale tumblr:

"“besides, it’s rude to talk about people that are listening, right? not everyone is as tough as my brother"

Given Papyrus lack of omniscience and that Gaster hear when people talk about him,i think the "brother"Sans is talking about is Gaster