I feel like a huge failure. It's hard to explain..
I don't think this is YOUR failure. In situations like that there is just no 'right' way to handle it. Your sister is an addict and there is no way to deal with her rationally - because she's not rational.
I lost my sister to drugs. She didn't fit he profile of your regular ol' drug abuser. She was a highly respected recovery room nurse. Good at her job and loved doing it. But things started changing. Her son was a very irresponsible young man. Got a girl pregnant and had a gorgeous, innocent daughter. But, he didn't take responsibility and my sister took this child to give her a stable home. Sister was also married to an alcoholic. All that pressure started to take its toll. She started job hopping. She told me she needed a job without so much pressure...she had too much on her plate. She took a night job so she could spend more time with the baby. One morning on the way home from work she ran off the road. She took a few weeks off and then changed jobs again.
My family (mother) has always been all about secrets and appearances. Many years ago I moved away from the family core and was basically out of sight, out of mind - and that was ok with me. But, I was also left out of the family secrets and didn't spend enough time with my sister to really know what was going on.
Turns out there was a reason behind the job hopping, a reason for the wreck, a reason for the 'couple of weeks off' that I never knew about until it was too late. Turns out that my sister had started using drugs...not street drugs...pharmaceuticals that she was pilfering from her jobs. She died at the end of her shift, locked in a staff restroom in the hospital where she worked. They got maintenance to unlock the door and found her dead with a syringe in her arm and her head in a trash can.
My point with all this is that I WISH I had had the opportunity to beat the crap out of my sister! If it would have knocked some sense into her, or have shocked her enough to make some changes I would gladly have done it. I would have done it just like you, just hoping it would steer her in another direction and protect her and those that now live without her.
I feel like a failure because I couldn't make things right. I feel like a failure because I couldn't save her.
You did what you could do at the time. You took a stand.