Hi, SmallTown. Welcome. Sorry for your trouble, but glad you found us.
This is a supportive site, and I am sure others will be by to welcome you.
Sadly, you cannot control your sister's behavior.
I recommend going to Al-Anon or Nar-Anon meetings.
The people there are or have been exactly where you are now. They can help.
Be the best aunt you can be to your niece. Look out for her, and do not hesitate to call Child Protective Services if you feel she is at risk.
I know you stated that you don't think much of them. My sib was with the organization for many, many years. I can tell you that he and his team worked tirelessly and endlessly to protect the children on their watch.
Addiction is the saddest thing there is.