Ford had a small lisp when he was younger 4-8. Not very long, he grew out of it. But it was on his S’s. He hated when his parents made him introduce himself to family and strangers, more than he already did (for the regular six fingers and shy reasons).
That’s when Stanley nicknamed him Ford. Later came names like Sixer, when his brother could pronounce it better. Stanford called his brother “Lee” most of their lives, when they were alone. Stanley most everywhere else, because he knew Stan hated others calling him Lee.
(Family nicknames, ya know? You can bet your bottom dollar no one outside of his siblings and Ma Pines called Filbrick “Filly”, but it was still a family nickname.)
So. Lee at home, Stan/Stanley to the world. Usually Stan. But Ford stuck with Ford for almost his whole life. Ford to the world. Stanford to the family. Sixer to Stan. It’s a family thing.
That’s why Shermie knows his brothers as Lee and Ford. It was a brother’s only deal. Sherman to the world, Shermie to the family. Stan couldn’t say Sherman fully (he spoke first), so Shermie started and stuck. Ford said “Thermie” for the first few years. He begged to let that joke die.
And so it continued. Samuel Pines. Born before Stan was kicked out. First to talk about “Little Sam-the-man”. It stuck. Rebecca, Shermie’s wife. Reb to the family. Rebel to her parents (she wasn’t really welcome home, there).
Mabel and Mason. Dipper, to them all. Mabel gave him the actual nickname, but at the hospital the day they were born, Stan was the first to say “Wow, what a mark! Looks a lot like you got a Little Dipper on your hands,” and made a few more assorted star jokes to cover up how much he already loved the precious babies.