The system is not keeping the child in foster care "forever".
It is protecting the child until a couple of drug addicts clean up their sh**.
They created this chaos and they alone can fix it. They won't do that by blaming the "system" for the negligence and abandonment their child has endured under their care and for the consequences they are now experiencing as a result.
I would be EXTREMELY reluctant to place a child back in a home until the addicts had a lengthy recovery period. And in recovery circles, that's one year minimum.
I agree with the social worker. Statistically, relapse is much more likely than recovery. Your sister will have to prove, PROVE OVER TIME, she will not do what she has already done to child in the past: neglect and abandon.
Sorry, but I can't sympathize with your anger. Too much danger for the child at this point.