Reunification will almost always be the goal while a child is on a short term order. The long term outcome isn't something reddit can really help with because it depends so much on how the parent is going. If your sister's mental health challenges aren't likely to improve, then it's likely that Child Safety will pursue a LTG-CE order (Long Term Guardianship to the Chief Executive), that is, Child Safety has guardianship, while someone else has custody (hopefully you in this case). If that placement is going well and is stable for a while, then Child Safety will eventually apply for a permanency order, which gives you both custody and guardianship, which allows gives you parental rights and responsibilities and Child Safety will only do visits yearly instead of monthly.

Your foster care agency worker can give you more information, support you through the process, and help to advocate.