Good news, we finally have a definite answer as to who that baby in A Tale of Two Stans is! According to Jeffery Rowe, one of the writers for Gravity Falls, the baby is in fact Shermie, the Grandfather of Dipper and Mabel and evidently the younger brother of Stan and Ford. A lot of debate has been going on over this saying that the math doesn’t add up as the baby today would still be too young to have grandchildren, making some people believe that perhaps Shermie was older than Stan and Ford and that the baby was his. However, although it’s a bit odd for a man to be a grandfather in his 40s, it’s not out of the realm of possibility. Glad we can finally put this one to bed! 

EDIT: Sorry to get you guys all excited for nothing, Jeff just trolled us by mistake. He actually isn’t sure if the baby is Shermie or not. I guess the baby will remain a mystery.