What is the ruling on looking at one's sister, then touching or fondling her to alleviate or remove the desire forzina (unlawful sexual intercourse)? And is masturbation permissible in this situation?

Incestuous desire is a heinous evil and a deviation from natural disposition. Merely looking with lust at anyone other than one's wife is forbidden, so how much more so touching or caressing them, especially if they are a mahram (unmarriageable kin). Whoever finds within themselves such desire towards their mahram must fear Allah and restrain themselves, cut off the causes of temptation, block the devil's entry points, distance themselves from everything that incites lust, and beware of being alone with their mahram. Practicing masturbation is not permissible.