My sister was sentenced to a term of incarceration but made work release eligible and stipulated to the county. Since she has been incarcerated she was told by the prison that she must serve six months before she will be allowed to start her job. Is this correct?
Actually, I am not certain that is in fact the law. The rule that you state seems somewhat county and/or judge specific. Generally work release allows a defendant to remain in the county for their sentence and to serve the sentence at the discretion of said county, including but not limited to going to work. Sometimes a judge will even allow a defendant to remain out of jail until they find a job then incarcerate them for the term of their sentence thereby making sure they do not go to jail until they are work release eligible.
If your sister has a job to go to I would have his lawyer appear back before the sentencing judge and file a motion that would immediately allow him to start work release.