What can I do for someone who is addicted to prescription medication?

I'm staying with my older sister for a while. My husband has a very demanding period at work at I thought it would be nice as I have two small children (a baby and a toddler) and have always enjoyed my sister's company. She was thrilled I was coming.
While I have been here, its become obvious that she has a dependency on Stillnox (a sleeping tablet renowned for being troublesome.) She is in remission for cancer which has left her with a few health concerns. Its never been a secret she takes these tablets as she says she has trouble sleeping and I've never really noticed when she had previously stayed at my home, any issues other than she liked to snack at night-time.
She had a few night shifts on this particular roster and I although I go out for most of the day when she needs to sleep, I come back for lunch during the heat of the day. She came out of her bedroom a bunch of times to get food, beers and even sat down and had a conversation with me and it took me a little while to realise she was asleep until she began talking rubbish and making no sense. I told her to go back to bed and she did, just like a sleep walker would.
I initially thought she wasn't aware of what she was doing and was worried as I knew you can't drink with theses tablets. I discussed it with her the next day and she thanked me and seemed surprised and even laughed at her antics (she had made a mess in the kitchen, and of course I related to her the conversation she had with me which established she was very much asleep as none of it was accurate.)
Over the next few days though, the behaviour continued and one particular day, I returned home for lunch and there was a tablet on the kitchen bench within reach of my toddler had I not seen it. She had made a massive mess in the kitchen, cupboard doors open, a beer soaked towel in the laundry from a spill and had downed the beers she gave me for Christmas. Of course I spoke to her again the next day and she admitted she had taken 4 tablets. We resolved I would dispense the tablets so she wouldn't take so many because she said she wasn't aware of dong it at the time.
I have discussed my issues with her doing this, (such as possibly still being under the influence at work, driving and having her nephews see her like this - she looks drunk and runs into walls.) This worked for a few days until she took two out of the packet and when I objected she was cranky at me. I have told her we won't be able to visit each other should this continue but doesn't seem to get it.
She has since gone out and obtained another prescription and continued on her merry way except this week she is just being sneaky about the drinking and seems to be only taking one at a time (I have checked the packet - I know where she hides it,) plus she has not had any night shifts so I'm not seeing it as much. I'm about to go home, I don't want to get into an argument about this with her but I think she has a big problem. I've scoured the internet for advice and can only find fact pages. She has no interest in being weaned off them (she stated one Dr had suggested this and another had told her to just live a healthy lifestyle.) She just changes Dr's until she gets a new script. I don't want my kids around this reckless behaviour and I am worried for her safety.