First of all you have not failed her, whatever she has chosen to do was her choice and not yours. She is totally responsible for choosing the lifestyle that she has chosen.
Addicts will literally drive you crazy. I used to stalk my exah, I broke into his phone and sent his drug dealers texts telling them I was going to call the police if I saw their number on my phone bill again. ( gosh that was a really dumb not to mention dangerous thing for me to do), I tracked his whereabouts, kept track of the odometer in his car to see if he drove further than what it took him to drive to and from work and at one point I monitored or at least thought I did, how many pills he was taking a day. I was a better slueth than Nancy Drew, true story 🙄.
None of the above resulted in anything positive, it only made me insane and it certainly didn't stop my exah from using. In fact he continued to use and just got better at hiding things which made me better at finding them. It was a crazy, insane time in my life.
I know you love and care about your sister, I can feel it in the words that you wrote but you cannot control her or her addiction. The only thing you can control is yourself and your reactions to the insanity your sister brings you.
Have you been to an ALANON or a Naranon meeting? If not I highly encourage you to find one that fits you and attend. Those meetings helped me so much when I was dealing with my exah. They give you the tools to put the focus back on yourself and not on th addict.
Also take a look at the posts here on this forum because the sistiations here will closely match your own. The strength and experience from other member who have gone through the same thing will carry you through the tough moments.