What to do with a sister on Crystal Meth?

I’m the second oldest to 5 girls. My youngest (19yrs) sister is a crystal meth addict. She is very moody and stays overnight elsewhere without letting anyone know. She comes back in the morning tired and sleeps through the day. She steals from my parents to feed her addiction. It’s killing my mom inside and we have all tried to help but she never follows through and always says that she’s off the drug but we found out she had been smoking it in her room under my parents’ roof. None of us live at home anymore because we are married and the fourth sister is in med school. My parents are traditional asian and we have never encountered this type of drug addiction. We get opinions of other parents that have troubled children but most have gone to get help. I don’t want to give up on her but is she a lost cause? We don’t even know how to get in contact with her. No cell phone and all her good friends are no longer her friends because of her addiction. If I find her, what do I do?