I'm an abused child and I'm scared for my life, I need advice ?

My mother and father died when I was 6.I went thru the foster care sister until my maternal Grandmother adopted my sister & I.She is NOT mentally stable.I am 15 now.She has never abused of my little sister,who is 12.But since we were little has hit me, mentally,emotionally and physically abused me every day.Since I was put in her care I have not left the home, except to go to school.I have been to a mall 4 times in my life that I remember, there are a lot of things I have never see.She does not even let me go get the mail for fear I will run away.She is an alcoholic and overly medicated.Since I began developing, she started calling me horrible things non stop.All she ever does is scream.She found out I have been in a relationship for months with my boyfriend,and when she saw him forbid me to ever see him again.Because he is tan, she is extremely racist.We're both Hispanic, yet she is EXTREMELY racist to every group except whites,and homophobic.She sickens me.I have no one to reach out to for help.And if I did and were not taken away she would kill me.She has attempted to before while drunk.And I am very afraid.No one at school or ect would even believe me.Because I have always lied and said I have the best mother in the world.I am too scared to tell anyone anyways.I have been having suicidal thoughts lately, and I do not know what to do.