What is my duty - other than advising - towards my sister who goes to university in un-Islamic attire, even though my father is aware of this and does not intervene, and what is the opinion of the religion and scholars on this matter?

Your sister going out without hijab is an abomination (munkar), and your father must prevent her. As long as you have advised her and your parents, you have fulfilled your duty. Indeed, enjoining good with the tongue is an obligation for those who cannot do so with their hand. Their lack of response will not harm you. Continue to advise her and supplicate for her guidance, and avoid sadness, as it brings no benefit and repels no harm.