I hope you find some comfort with everyones support here, we all understand a broken heart that comes when you love an addict. My husband (ex) was the addict in my family, I had a hardtime getting a grasp around "he doesn't love me or he would not be doing this", we have two sons together and onething I did know was, he loved those boys with every inch...once those drugs got a hold of him he would not stop even for them.
It is so hard to understand bottoms, my exah did robbery X 3, there was a very good chance that he was going to be going to jail for a longtime, he got off with a house arrest. I was so happy, thinking this scared him so bad, it was going to be the end of it. While I was talking with his lawyer after this, he told me to get out of this marriage, that my husband was a serious bad addict and he could guarentee me that this was not going to be the end of it...no no I said this really scared him straight, hit his bottom! A week later I recieved a call, he was back in jail, broke his house arrest, was at a crack house at 2 in the morning. So now he has lost his family, home, business, been in jail, overdosed, been stabbed, hit by a car purposely, living on the streets and still yet to hit his bottom.
I am sure you have done everything possible you thought you could to make her stop, it is OK to rid yourself free of it, feel no guilt, do what ever you need to do to make peace in your life. It is very wise as the others have mentioned, to tell your sister not to inform you about her goings on. To bad your sister would not break the contact with her as well, but that is for her to decide.
Take Care,
Rose