Firstly, for period 3'ers, my math was bad (not the first time) - Fyodor was 15 when mom died and not yet 17 when his father died.
As for our blog prompt:
Re-read the letter Raskolnikov receives from his mother. Why do you think he is so angry at the news that his sister is going to marry Luzhin. Our brief discussion and Dostoevsky's bio should weigh upon your answers.