My mother hates me

I am 17 yrs old and I am not allowed to have a boyfriend but I have and weve been together for 3 yrs already. They dont know anything about us untill last year where they saw us together we talked about it and finally they allowed me. I have a sister which is 16 yrs old and did the same thing. Now my mom is so mad at me she's shaking while telling me that if something happened to my sister and if I get pregnant, I better start packing my things. I feel like its so unfair for I dont control my sister's brain. I knoe Im a bad influence for her but still.. Its here mind, her own decision. And I cant get WHY the hell cant i have a boyfriend. I dont know what to do.