Been there, done that, got the t-shirt and postcards. Some people can't seem to help themselves and head down the path of no return. I hope and pray that your sister's story turns out better than my brother's. He ended up committing suicide. Someday I will post about it, but for now, it is still too raw...
Remember that you are but one person and it is impossible to save everyone.
You have my condolences, we had one in the family that did the same last year. :(
It is hard medicine to swallow. Know that you are not the only one who walks this path to hekp family.