What is the ruling on one who used to steal glances at his sister while she was bathing or changing her clothes when he was seventeen years old, then repented, but still remembers seeing her naked whenever he sees her now?

What you committed by looking at your sister necessitates sincere repentance, which includes abandoning the sin, regretting it, and resolving never to return to it. Looking at 'awrat (private parts) is forbidden, and it is even more severely forbidden with regard to maharim (close relatives whom one can never marry); in fact, it is among the grave sins. Among the requirements of repentance is not to dwell on these thoughts or remember what you used to do. Rather, you must cut off such thinking and occupy yourself with the remembrance of Allah, the recitation of His Book, and supplication that Allah remove that from you. It is also advisable to avoid being alone with your sister if you fear falling into temptation.