I was skimming Reddit and saw someone post a theory version similar to the one I previously posted. There was a difference here, however. Well, more of an expansion. It makes sense the more I think about it.
If Stanford is actually the twins’ grandfather and Stanley is the twins’ great uncle, then wouldn’t that mean that Stanford’s name is actually Stanley and Stanley’s name is Stanford? Let’s not forget that Stanford had the Stanmobile (which has a license plate reading “STNLYMBL”) long before his brother went to Gravity Falls, Oregon. My only issue with this argument is that the one time we see the Stanmobile in a flashback, we can’t see what’s on the license plate.
So maybe, in addition to Stanford actually being Mabel and Dipper’s grandfather, the brother fans have been referring to as Stanley all this time is actually Stanford, and Stanford is Stanley.
I’m about half and half on this expansion to the theory.