Why did she grow abhorrent towards her little sister? How did the relationship grow in subsequent years?

Helen was jealous of her little sister Mildred for a long time. She felt all focus shifted to Mildred rather than her,

For a long time I regarded my little sister as an intruder. I knew that I had ceased to be my mother's only darling, and the thought filled me with jealousy.

As Helen matured, she grew to love her little sister,

..but once I discovered my little sister sleeping peacefully in the cradle. At this presumption on the part of one to whom as yet no tie of love bound me I grew angry. I rushed upon the cradle and overturned it, and the baby might have been killed had my mother not caught her as she fell. Thus it is that when we walk in the valley of twofold solitude we know little of the tender affections that grow out of endearing words and actions and companionship. But afterward, when I was restored to my human heritage, Mildred and I grew into each other's hearts, so that we were content to go hand-in-hand wherever caprice led us, although she could not understand my finger language, nor I her childish prattle.