HI Lucie........Welcome.
My 26 yr. old daughter was the addict in my life. She walked down a very bad path for about 6yrs. She is doing well now, but it didn't happen overnight. You cannot control what your sister does and neither can your family. Addiciton is a powerful thing and if we had any control over it no one would be on this site. Just remember you didn't cause it, you can't control it, you can't change it.
You can however control yourself. It is about taking care of you. All of the worry in the world will not make your sister stop. You are young and have a good life ahead of you. Don't let anything get in the way of that. If your parents are putting too much on you with all of this, it is time to tell them.
I'm not saying you shouldn't be there for your family, I am merely telling you not to lose yourself in the process.
Read all that you can about co dependency and enabling.
Stick around here, there are a lot of good folks on here that will be more than happy to listen to you.
Hugs............Lo