[That recognition releases some of the stress from his smile, if Erwin already knows of him. Although, that means Erwin is another person who knows of Papyrus before meeting him, thanks to... whatever Sans has been telling them. Good to know Sans is already starting to get to know literally everyone again.]

Yes, Sans is my brother. And his tree... it's pretty bright and obnoxious.

[It's on the side of the Undertale house that shares yard space with the Knochenmuses, and it's clear as daylight - and sometimes as eyeglaringly bright - that Sans specifically put it there to be annoying.]

But not as bright as mine! He'll give up decorating his eventually. Nyeh heh heh heh heh!

[Papyrus breaks into some outright Saturday Morning Cartoon villainous cackling, as this is not a one-sided prank, and he is escalating the situation.]