Mildred was Helen Keller's younger sister, born when Helen was five. As most older siblings do, Helen regarded the newcomer as an "intruder." Mildred was, to Helen's mind, competition for her mother's affection.

Helen, at first, felt that Mildred was her mother's "only darling." Helen was jealous that Mildred seemed to forever be taking the place that Helen regarded as her own on her mother's lap. At one point, when Helen found the baby in her doll's cradle, she overturned the cradle in a fit of rage that might have killed Mildred if her mother had not managed to catch the baby.

But, also like most children, Helen adjusted to her new sibling and made the best of the situation. The two became friends and went around together, though they couldn't understand each other.

Later, after Miss Sullivan arrival, Mildred learned manual writing so that she could communicate with her sister. Mildred sometimes accompanied Miss Sullivan and Helen when they went out. In one exciting story, she saved them from a speeding train.

Mildred Keller was the younger sister of Helen Keller.  She was several years younger.  When Mildred was a baby, Helen had a treasured doll.  She often rocked this doll in a cradle.  One day, Helen found her infant sister in the cradle instead of her doll.  She became angry and flipped the cradle over.  Fortunately, Helen's mother was nearby and caught the baby before she fell.  

Later, when Helen was older, she and Mildred would go out into the woods and gather persimmons and nuts.  They also liked to explore and even got lost in the woods.  Years later, when Helen was at Cambridge, Mildred came to the school as well.  For six months, they attended school together.  

Helen explained that despite her early jealousy, she and Mildred "grew into each other's hearts."