Is it permissible to look at a beautiful actress or to masturbate to avoid having a wet dream about one's sister, and what is the guidance of Sharia in such cases?

It is by Allah's grace that He guides a Muslim to obedience and grants him a self-reproaching soul. Wet dreams are a natural occurrence for men and women, married and unmarried alike. There is no sin in them because they are beyond the sleeper's control, and wet dreams are not an indicator of wrongdoing during wakefulness; rather, they may be from Satan to lead a Muslim into indecency.

To avoid bothersome wet dreams, it is recommended to perform ablution (wudu') and recite Ayat al-Kursi before sleep. One should also avoid dwelling on desires and forbidden gazes, and refrain from eating excessively before sleep. The fundamental solution is marriage, and for those unable to marry, fasting is the Prophet's (peace be upon him) recommendation. A Muslim must be careful not to cast forbidden glances at his sister or other women, lest he fall into the forbidden or be affected by it in his dreams.