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.