Kitty realizes that Tanner is illiterate when she suggests that he write to his sister.
Tanner is an orphan. He was hoping to become an apprentice, but ended up being a servant. Kitty also had higher hopes, but finally the two married. Kitty is aware that Tanner has no education, and she helps him learn.
When Kitty asks Tanner about his family, he says he has one sister and she probably thinks he is an apprentice. She asks if he has sent word lately, and he replies that he has not.
Best to write to her now, anyway, said Kitty, and tell her how it is between us. … I’ll think it over, he said. Kitty realized that he could neither read nor write. (p. 362)
Although Tanner is not educated, he is humble and honest. He really does love Kitty, and he tells her to wait for him. He does not rush into the marriage, but asks her to wait so that he can save up money. All in all, they are happy and eventually one of their oldest daughters becomes a lawyer.