What letter does Olga's mother write to her on her 13th birthday?

Olga receives a letter from her mother on her 13th birthday. Her mother writes to tell her that she is going away because she needs to focus on her work and will miss her birthday. She tells Olga that she wants her to be strong, behave, and not fuss like a child. She will not be alone. She will have her brother, her grandmother, and her mother's sisters and brothers.