Why are my mother and sister so horrible?

well my dad just left i only recently found out that my sister is only my half sister so she gets alot of the attention she is two years older than me in turning 17 this year.....well my mom and sister get along well but i'm usually left out of things...when they get mad they say the same horrible insults mom tells me how fat i am and how much skinnier she is than me jazmin (my sister) agrees with everything she says and makes everything sound like i'm this horrible person like she will back mom up and say she wanted to upset you she wanted to embarrass you, and because my dad left and he's my dad they say things like i'm just like him miles shouldnt breed because they all turn out to be losers and that she wishes i just died to be put out of my misery that i'm ugly that i hae no one they threaten to leave me after i finish school and its really horrible please what do i do