I believe you are talking about Louise's struggles regarding her love for Captain Wallace and her feelings for her sister.
In Chapter 11, the Captain's house is destroyed by a hurricane, and he and Louise go to see the extent of the damages. Louise reacts strongly when the Captain hugs her in the boat, thinking she's falling in love with him. In the next two chapters, as Louise struggles with her love for Captain Wallace, her grandmother makes her feel worse by saying Louise is having an affair with him.
Louise's feelings toward her sister are also reflected in these three chapters. Louise gets upset when Caroline, her sister, uses her hand lotion and gets violent toward Caroline, reflecting her bitterness and jealousy of her sister for so many years. When Caroline suggests that Captain Wallace marry Trudy, Louise is even angrier at her.
These chapters are the culmination of Louise's hatred and bitterness toward Caroline. Louise feels she's been denied the love and attention of her parents and friends because of her sister. Her grandmother's cruelty only makes her resentment stronger. Her friend, Call, and Captain Wallace even seem to like Caroline more than Louise. It's only after Caroline goes away to school that Louise begins to understand the actions of the people around her. With the Captain's help, she learns how destructive her feelings will be if she can't come to terms with them.