All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Masturbation is forbidden, its ruling and the ways of getting rid of it are mentioned in Fatwa 81150, so please refer to it.
What you mentioned that you desire your sister for sexual intercourse is forbidden and an evil matter. So, it is forbidden for you to look at her and be in seclusion with her when you desire her, or when it is most likely that you will do so. Looking at female Mahaarim (non-marriageable relatives) with a desire is forbidden like looking to any other marriageable woman, whether this desire is for sexual intercourse or just lustfully enjoying oneself by looking at her. The scholars said: 'It is absolutely forbidden for a Mahram or else to look with a desire to any woman except his wife.'
One of the Indian Fataawa reads: "If it is known that a person desires his female Mahram or that she desires him if he travels with her, or stays in seclusion with her, or he is pretty sure that this is the case, or he doubts that this may be the case, then it is forbidden for him to stay in seclusion or travel with her."
So, you should fear Allaah and ask him to repel from you these desires, and we ask Allaah to guide you to the right path.
Allaah Knows best.