Evidence of Stanley
Grunkle Stan
Stanford's title to Dipper and Mabel is Grunkle, short for great-uncle. He also shares their last name. This means, twin or no, that he must have a brother, who would be Dipper and Mabel's paternal grandfather.
Stanley Mobile
Stanley's name comes from Stanford's car's license plate. It says "STNLYMBL" which is likely short for Stanley Mobile. This cannot be explained if Stan's real name is Stanford.
Misc
In The Legend of the Gobblewonker, Stanford reveals near the end of the episode that he has to wear an ankle bracelet / tracking collar. He is not seen with it for the remainder of the episode or season. This could show that Stanley was a tinkerer, and taught his brother some of the skills.
Stanford must have loved Stanley deeply. In Headhunters, some say that Wax Stan reminded Stanford of Stanley, and this is why he was so fond of Wax Stan.
