Foster mom of three years here, well experienced with CPS. Like biminiblue said, document everything. Do welfare calls on her if you suspect she's not conscious. When you do these, call 911 and say "My sister has an addiction to prescription medications and I haven't been able to reach her in X amount of hours/days- could you send someone to check on her please?" They'll do it. And if they find something alarming, a call to CPS will take place. You can also call in reports to CPS if you know that neglect is happening, or if your younger niece is being driven while mom/sister are under the influence.
We just adopted two of our foster children a couple of months ago. Before they came to our home and into "the system," there were TEN investigations by CPS done. Family was reporting, nurses, doctors, neighbors, boyfriends, etc. But CPS's hands were tied. In order to remove a child from a situation, they have to prove to the presiding judge that there was irrefutable evidence of abuse or neglect. As such, they would show up at mom's place and all signs of drugs would be gone. Apartment was tidy. Food was in the cabinet. There were 'logical' explanations for the bruises on the kids. Until one day there wasn't. It took 3 years of calls from family members and community members to finally catch her neglecting those babies, but it worked on the 11th call. They finally found enough evidence to bring them into a foster home (our home), and now they've been adopted forever.
Don't stop advocating. You're doing the right thing.
BellJar