Welcome Lucie,
This disease of addiction can really tear up a family. I did it to my mom, dad, and brother when I was your sister's age. Now my daughter (age 19) is doing it to me, my husband, and my poor mom (who had to go thru it twice). We addicts (when using and NOT in recovery) are a driven, obsessed, lying, stealing, conniving, manipulating, pathetic, and totally self-centered bunch of people. The only thing i responded to when when my mom and dad changed the locks on our house and let me ride out the consequences of my drug addiction. Once I was ready for help, though, they were always there for me. I am trying (though its very hard)to do the same for my addict daughter. I do see her, take her out to lunch and talk, take her to the HIV clinic for appts, stuff like that. She knows I love her and will always be there if she wants help. But I don't give or lend her money, she can't live at home unless she has been clean for a month, and I pretty much take everything she says with a large grain of salt.
I hope you can find the identification you need here and we can help you get thru this.