I've been wondering this for a long time. Who do the Pines children think their grandfather was if they believed Stan to be their uncle? They must have known he had a brother because he is their uncle and somebody has to be their grandfather. Did they think he was long dead? What does their dad know of all this? He…

The biggest evidence for Stan having a brother is that he had to have a sibling to be an uncle. And for Dipper and Mabel to have the Pines name, then he would have to have a brother.