Favoritism at work

My sister has been employed at Pep home (westonaria) since august 2016 as a casual worker

Throughout the time she's been working there, a new manager was appointed with some of her stuff from a different branch. Favoritism has always been a issue towards certain individuals from her stuff ranging for who gets called to come work and who gets a contract

Just on Thursday they were interviews for a permanent position for some of the casual worker, my sister was one of them but her interview was unsuccessful

How does a person go about to query such?