Dude, this chapter is written by GirlOfManyFandoms.
So, you probably guessed by all the "duding," this is on Soos. Well, there was also the subtle hint in the chapter name. Real subtle, right?
So this has to do with last names, particularly the last name of Soos. Everyone in Gravity Falls seems to have a last name announced. Even Robbie V. But Soos... is just Soos.
Okay. Here me out! Soos is a distant relative of the Pines. We know he started working at the Shack when he was twelve, and this implies that he is family. I don't know about you, but here in Mississippi, there is a law or whatever that says someone can't work somewhere unless you are sixteen. If you are younger, it has to be a family business. See my point?
I should also mention that he really enjoys spending time with Dipper and Mabel. Most employees don't want to babysit their boss's kids. He does it willingly.
The rest of my theory lies solely on episode 19, "Dreamscapers."
When Dipper agrees to help Stan, he says, "Fine, get ready, guys. We're about to journey into the most horrifying, disturbing place any of us have ever been: our uncle's mind." Now, I know this is lame evidence. I mean, Dipper could have said "our," referring to Mabel and himself. But nonetheless, he did say "our."
Usually, when I am talking about a family member that is not related to everyone in the same way as me, I don't say "our." Like if a neighbor, my cousin, and I are hanging out at my nana's. I'll say something like, "Nana wants us to go outside." She is the nana of my cousin and me, but not the neighbor.
If Soos isn't related, why didn't Dipper just say, "... any of us have ever been: Grunkle Stan's mind."
And also, Bill Cipher said, "Ah, Stan's family, we meet at last! Question Mark, Shooting Star, Pine Tree..." He added Soos (Question Mark) in when referring to Stan's family.
Who knows? I mean, Soos may not be a Pines. He may have a different surname. But I stand by him being related to the Pines family.