adopting my sister. . .

I am seeking for some advice, and am hoping that one of you may have the answer or at least some helpful input. I am 18 years old, born in Arizona and raised in Oregon. My mother raised me without the help of my father who walked out on me as a baby. My father went back to live in Nogales, Sonora where he could feed his drug addiction at lower costs than he could in the States. When I was 10 years old he had a daughter from another woman. She was a drug addict and abandoned my sister, leaving her with my fathers mother. My grandmother has raised Sarahie (my sister) but her health is faltering and my father has been in and out of rehab for years now, never making much progress. SarahieҒs mother has also not quit using drugs and I am the only other immediate family member besides my uncles who have kids of their own and dont want to take responsibility of her. As much as my grandma tries she is not giving Sarahie the stability that she needs and doesnҒt have the resources to provide for her. I am wondering do I have any right to take guardianship of my baby sister. And if I do how do I go about becoming her guardian? Also the major factor is her citizenship. I was born and raised here but she was raised in Mexico and due to the fact that my father is not an American citizen she had no citizenship here. What do I do? Would I have to adopt her? I want my sister to have the best life possible and I want so badly to make some sort of difference for her. If I were to adopt her would she get all the opportunities she deserved? Please help me!