There is nothing wrong with your sister working in the students’ residence, especially if it is in your city, but if it is in another city then it is not permissible for her to travel there and back except with a mahram, but it is permissible for her to stay in that residence without a mahram, because what is required is only that she be accompanied by the mahram on the road. The same also applies to the female students who live there. Your sister is not expected to examine each student to find out whether she came with her mahram or travelled without him, rather it is sufficient that her work in the residence does not involve anything haraam. Administration, guard duties, cleaning and so on are all permissible and there is nothing wrong with that, in sha Allaah.
https://www.google.com/amp/s/islamqa.info/amp/en/answers/85045