Sincerely (Yours) - Tezy (Ao3)

Description:

(Late 19th century AU)

Frank is the son of a merchant, with no money, and no title. But he's smart, and he enjoys teaching, so when the opportunity arises for him to help with the two children of a particularly wealthy family, he decides it is worth the risk. The children are precocious, outspoken and they quickly become his favourite students. Their slightly peculiar uncle, however, is the real mystery for him.

————————————————

I melt every time I read this one. I absolutely love it.