I am a victim of parental favoritism.HELP

Ever since I was a little girl, I knew that my mother loved my younger sister more than she loved me. The signs are obvious: my sister is given a higher allowance; she is praised even though I have straight A's while she can fail a class; she is never punished for her misgivings, but rather, I am; and, when my mother speaks to her, my mother's tone is softer than with me. As a little girl, my mother used to physically beat me as a scapegoat for whatever misfortune she had and later, she would seal me into a little, dark box in the basement for hours. My father was my only confidant; he was the one who saved me time and time again. Now, after my father's accident, I feel completely alone. I contemplate suicide sometimes, to the point that I can feel death on my lips.