Helen Keller lived in a world of "silence and darkness" after her illness at a very young age. Although she fills her world with new experiences, there are many things that she has no capacity to understand, share in or appreciate such as "tender affections that grow out of endearing words and actions and companionship.
As a result, Helen feels that her younger sister Mildred is nothing more than a nuisance who steals her mother's time and affections. During one particular incident, Helen finds Mildred sleeping in the cradle that she uses for her beloved doll and tosses her out. Luckily Helen's mother is not far away and Mildred is unharmed.
Mildred is firmly entrenched in Helen's "heart" after Helen becomes "restored to my human heritage." Clearly her mother made Helen understand the difference between having a tantrum and throwing her doll around and throwing her sister around.
After that event, Helen becomes close to Mildred and later they go on nature walks together and are together when they have a close encounter with a train whilst trying to find their way home.