Everybody is trying to debunk the Stan twins theory but can I just put out there;
Grunkle Stan has to have /some kind of sibling/. Be it a twin brother, twin sister, or just a regular brother or sister. He /has/ to have one. Because he’s Mabel and Dippers Grunkle- aka Great Uncle.
If he didn’t…
I really wish more people would bring up this point.
Also: He would have to have a brother, because Dipper, Mabel and Stan all have the same last name, and the only way that is possible is if Stan had a brother, who be the twins’ paternal grandfather, because the male takes the last name
Unless I’m forgetting some sort of circumstance, that is the only way they can all have the same last name