We all KNOW he had a brother at some point. Grunkle stan never had kids as far as we know. But his and the kids last names is Pines. SO it would mean he had to have a brother who had a son who then had dipper and mable. It is persumed that all the mystery stuff was started by Stanfords brother Stanly which caused him to disapear around the time his some was born. Wich would mean that dipper and mabbles dad never knew his own father and dipper and mabble now nothing about their grandpa. But they know their grunkle stanford.
Actually it might, or might not be he's brother that hasn't been revealed. He has minor differences in his design that are making that possible to say. Like he's nose shape that is slighly different. And shape of his glasses that are really similar to that same glasse that Stan was holding in secret body swaping carpet room that he took from a shelf under a callendar. This show has lots of interesting secrets.