What is the ruling on accompanying one's sister, who wears un-Islamic attire, to drop her off or to go out with her for leisure, knowing that both parents acknowledge their mistake in allowing her to do so?

You are not permitted to go out with your sister, who wears tight clothing, for recreation or any other purpose, unless you fear that evildoers may accost her, or that she may be exposed to harm. In such a case, you may go out with her, while exerting your utmost effort to advise her and forbid her from doing so.