Is it permissible to masturbate to avoid watching pornography and lustfully looking at one's sister?

The forbidden acts mentioned in the question require striving against one's self to abandon them. It is not permissible to commit some of them to get rid of others, for committing one forbidden act leads to another. Allah commanded the guarding of one's private parts and eyes, for sight is the beginning of temptation. Lowering the gaze and guarding one's private parts are purer and more chaste. A Muslim must be a guardian of himself and constantly guard his four frontiers: glances, thoughts, words, and steps. The cure for these forbidden acts is to seek refuge in Allah through supplication, to be mindful of Him, to remember His constant observation, and then to strive for marriage. If one is unable, then he must fast.