Mildred Keller was Helen’s younger sister. Before her birth, Helen used to get all the attention from her parents but after Mildred’s birth, things changed which made Helen jealous of her. When they grew up, they became friends. However, she could not understand Helen’s finger language. Once at the Fern Quarry, they get lost in woods. Mildred pointed towards the trestle and they walked over it. They climbed a moving train and returned to their cottage. This shows her alert behavior.