How can one deal with the devil's whisperings that lead to forbidden desire towards a married sister, especially with attempts at harassment and no objection from the sister?

What you have mentioned is a serious matter, and you both must be careful not to fall into it. You must not be alone with your sister, nor sleep with her in the same bed or even the same room. It is not permissible for you to look at her or touch her. She, too, must preserve her husband's sanctity and prevent any causes of corruption. Adultery is one of the gravest major sins, and it is even more severe with unmarriageable kin (mahārim). Its perpetrator is punished in Islam by death, as the Prophet (peace be upon him) said: "Whoever commits intercourse with a mahram, kill him." A brother is commanded to protect his sister, not to violate her honor.

Advice: 1. Fear Allah and maintain prayers on time in congregation at the mosque. 2. Accompany the righteous and attend gatherings of knowledge. 3. Marry if you are able. Otherwise, you must fast, as the Prophet (peace be upon him) said: "O young people, whoever among you can afford to marry, let him do so, for it is more effective in lowering the gaze and guarding chastity. And whoever cannot afford it, let him fast, for it will be a shield for him." 4. Remember Allah's watchful eye and the dire consequences of adultery, and Allah's prohibition of it, as He says: (And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way). 5. Increase in supplication and seek refuge in Allah to avert indecencies, purify the heart, and guard chastity. 6. Occupy your time with acts of obedience, repentance, and seeking forgiveness for what you have committed, and pledge not to repeat it.