Mildred Keller is Helen's younger sister as explained in The Story of My Life. There is not a lot of information on Mildred in the autobiography but the reader does learn that, after an event when Helen tips Mildred out of a crib, the two sisters do grow "into each other's hearts"(ch 2). Helen is jealous of the attention that Mildred gets from her mother - attention that used to be directed to her. One day she notices that Mildred has been placed in a crib reserved for Helen's beloved doll, Nancy, and it is just as well that Helen's mother is on hand to avoid catastrophe.
One year, whilst spending the summer at the family's cottage in Fern Quarry, Mildred tells Helen all about what she sees - the train tracks and cows straying on to them, and the trestle. Helen and Mildred go walking in the woods with Miss Sullivan but unfortunately they get lost. It is Mildred who spots the trestle which allows them to find their way back but not before they are almost run over by a train. When they do return, the family is out looking for them as it is so late.
A poignant moment for Helen when she realizes that she and Mildred will be able to communicate more effectively is when Helen learns to speak and no longer feels "dumb."The most important thing on Helen's mind is that "My little sister will understand me now."(ch 13) Helen's family are delighted at her achievement with Mildred taking Helen's hand and dancing.
When Helen goes to The Cambridge School, Mildred also attends and the sisters are inseparable "for six happy months "(ch 18) although they are eventually withdrawn from the school after a disagreement between the school and Helen's parents.