Why does Olivia write to Marcia in Section 16?

Olivia writes to Marcia in Section 16 because she is upset that Mrs. Crawford and Mrs. Minnies express a low opinion of the Nawab. They blame him for not preventing the violence of the suttee death. Olivia must hide her growing feelings for the Nawab, and so refutes their accusations only briefly. She then takes out her frustrations and expresses her true opinions in the letter.