Generally, once the court has given a stay order means, the process will be stopped till the court case is finalised and a verdict has been given. Further development will depend on the court's verdict. If it dismisses the case filed against selection, the employer will send the appointment letters and then she can join. If the court verdict is given in favour of the applicant, the whole process will be cancelled and fresh process of recruitment will start. Then your sister has to appear for the whole process as per the notification. One has to wait for the court case finalisation.
Sometimes the case in the court may be not about the selection process and it may be disqualification of a particular candidate or related to an individual. In such a case, there is no point in waiting for the finalisation of the case. In such a case, you have to approach the court for vacating the stay for all other people who are unnecessarily getting a loss.
If you know some other candidates who are also waiting for an appointment like your sister, all of them can go to the court. For this, you may require the help of a good lawyer who is well versed in such cases.