Reading "Crime and Punishment" and noticed Raskolnikov addresses his sister with вы, even mentally:
I remember VM saying something about addressing your parents with вы. Does the same thing go for siblings? The weird thing is that he's on a ты basis with Razumikhin.Дунечка, милая, ведь я знаю вас! Ведь вам уже дватцатый год был тогда, как последний-то раз мы виделись: характер-то ваш я уже понял.
Besides this, I noticed they use прощай a lot(even when they intend to see each other again). The letter from Raskolnikov's mother makes an interesting distinction:
Прощай, или, лучше, до свидания!