In the flashbacks Stan has in “Dreamscaperers”, if you look in the background during the wrestling bit, you can see another boy with the same haircut/color and signature Pines cowlick with his face buried in a book.
Also, in Stan’s flashback in “Boyz Crazy”, he has a much different chin shape than the man we saw in “The Time Traveler’s Pig” that peeked out of the door of the Mystery Shack of the past. Stan as we know him does not have a dimple in the middle of his chin (or “butt-chin”, as some have taken to calling it).
Then there’s Stan finding and later looking contemplatively at the glasses he takes from the abandoned room in “Carpet Diem”, which resemble the glasses of the man above from TTTP (and the glasses on the wheel from the opening credits). (Note: Outlined images originally from prettyinpwn).
Pretty obvious in hindsight that he sealed up that room after his brother disappeared (presumably into the portal). Since the symbol on Stan’s fez is already on the wheel, we can now assume the glasses to be representative of his brother (still no goddamn idea who the hell the llama represents, though, since there’s been no association between llamas and, say, Pacifica).
While this may be unrelated (but probably isn’t), the license plate on Stan’s car says “STNLYMBL” (short for “Stanley mobile”). Grunkle Stan’s name is (allegedly, anyway) StanFORD, not StanLEY.
Also, to be a great uncle, he has to have a sibling.