I've made the BEST idea. To start, Stan has a brother. No big shocker, and I have proof! Mabel and Dipper's last name is "Pines," as well as Grunkle Stan's. Think about it, Stan is their Great Uncle, and a Great Unlce is the brother of a grandfather. Because last names are pasted down through males, it go Grandpa Pines to Mr. Pines to Dipper Pines.
That's allI got currently, but if you have any ideas, let me know.