What do we learn from Raskolnikov's letter from his mother?

Raskolnikov's mother, Pulcheria Alexandrovna, relates her personality and her priorities in the letter. . She is devoted to her son, even ready to accept her daughter's self-sacrifice for his benefit. Her letter introduces the subplot of Dunya's engagement. Raskolnikov is upset by the letter and feels the need to get out of his flat after reading it. It is revealed that Raskolnikov has not seen his sister and mother for three years.