Years ago I called Children's Aid (Canadian CPS) and had them check on my granddaughter because I heard from my son that they had both relapsed, were using needles and had people coming and going at all hours.
They checked and took the child to a "safe house" where nobody including me knew where she was. The child's mother soon got back on the wagon (she had been clean 7 years prior to the relapse) and after 6 months of "doing the do things in recovery" they gave the child back...my son was no longer there.
Someone has to be the voice of the child. By all means call and report what you suspect. They can check it out. Even if your sister never speaks to you again, you very well may have saved that child's life.
Hugs and God bless the child.