What is Helen’s relationship with her sister Mildred like?

Although Mildred, Helen Keller's younger sister, could not understand Helen's finger language, they were close when they were little. Even at a young age, Helen wanted to communicate with Mildred and wrote her a letter in Braille (which Mildred then tried to eat, much to Helen's consternation). Mildred served as Helen's eyes and even once saved her from an oncoming train. They spent a lot of time together, and Helen wrote Mr. Anagnos letters about playing with Mildred, stating, "I love to play with little sister." She described her sister in a letter to Miss Moore as blue-eyed, golden-haired, and sweet-natured. When she studied in Boston, Helen intended to teach Mildred the languages she had learned when she returned home. She was also thrilled to learn how to speak, in part so she could speak with Mildred. Later, Mildred came to Cambridge to study with Helen at Mr. Gilman's school. As time went on, Helen learned new ways to communicate, and she was motivated in part to do so to better communicate with her beloved sister. 

Helen's relationship with her sister, Mildred, changed over time. When Helen was a little girl and still unable to communicate, she almost injured her baby sister in a fit of anger. Helen usually kept her doll in a cradle. One day, she discovered her infant sister sleeping in it instead of her doll. Helen was enraged, and she tipped the cradle over. Her mother caught Mildred before she fell to the floor.
After Miss Sullivan came and Helen learned to communicate, her perception of life changed entirely. She saw the world in a new way. Helen began spending more time with Mildred. They enjoyed exploring the outdoors together.
When Helen was at the Cambridge School, Mildred joined her there for six months. During that time, they were inseparable. Helen later reflected on her relationship with her younger sister: "When I was restored to my human heritage, Mildred and I grew into each other's hearts, so that we were content to go hand-in-hand wherever caprice led us" (The Story of My Life, Chapter II).