I don't know where she is exactly, but I might be able to figure it out if she's posts more pictures. She traveling around. Her dad isn't involved. She is going to school.Do you know where your sister is? Do you know how to contact your niece’s dad? Is your niece attending school?
Three suggestions: One, contact an attorney in your area for advice and direction; Two, contact the school/school counselor at your niece’s school to inform them of the situation; Three, contact CPS and report your sister for abandonment.
I don’t see “do nothing” as a responsible option.
I don't have money for a lawyer and the other options mostly just seem like they'll all just end with her in foster care again, but I guess maybe that is really the only option.