Should one continue to advise a sister who wears immodest clothing, or should one stop in order to avoid the wrath of parents who consider such advice to be a form of disobedience to them?

Your sister is not permitted to wear transparent clothing or clothing that describes her body, even in front of her mahrams (unmarriageable kin) or other women. The matter is even more severe if she leaves the house dressed in such a way. You have done well in advising her, and you should continue to do so as long as the advice is beneficial. Choose appropriate times, and stop if it causes anger to your parents or makes her more defiant. Know that making abundant du'a (supplication) for her guidance is important. You may shun her if that would be beneficial, otherwise, strive to win her over. Remind your parents of their responsibility towards their children, and that they are a trust, especially your father, as he is the guardian and must be firm with her.