A young man is faced with desires when awake and has wet dreams about his sister!

Firstly:

By the grace of Allah, may He be exalted, towards you, He has enabled you at this age to worship Him and obey Him. By His grace towards you, He has blessed you with the ability to rebuke yourself when you fall short in obeying Him, and when you do something that it is not appropriate for you to do. These blessings from the Lord, may He be exalted, require you to give thanks in your heart, in your words and in your deeds, especially when you see young people around you of a similar age who go with the flow of lust and are drowning in a sea of misguidance and deviation.

Secondly:

You must understand that wet dreams happen to both women and men, single people and married people. They may happen to one who recites adhkaar before sleeping and to one who does not do that. There is no sin on a person for what he sees in his dreams, because he does not have any control over it or any ability to ward it off. The Muslim is not accountable for such things, just as he will not be rewarded for what he does in his dreams of acts of obedience and worship. There is a saheeh hadith from the Prophet (blessings and peace of Allah be upon him) which states that the Pen of accountability is lifted from “the sleeper until he wakes up.”

So you should not feel anxious about what you see in your dreams. What you should worry about and be concerned about is what you do when you are awake, and this is what you suggested as a remedy for the problem of having wet dreams about your sister. Both of your suggestions are haraam and it is not permissible for you to do them. It is not permissible to look at pictures of actresses, singers and other attractive girls. For information on the prohibition on doing that, please see the answer to question no. 1774. Similarly, it is not permissible to engage in the secret habit (masturbation). For information on the prohibition on doing that, please see the answer to question no. 329. In addition to both actions being haraam, they will never avail you anything, and they will never prevent you from having wet dreams. We mentioned above that wet dreams happen even to married people and those who fulfil their desires in permissible ways.

You should understand that looking at your sister with desire is emphatically forbidden. In addition to being forbidden, it also has a strong connection to what you see in your dreams. It is well-known that some of what is seen in dreams is the outcome of the thoughts that the person thinks when he is awake. This means that looking at your sister – or any other woman – in haraam ways and thinking about sex before going to sleep may lead to seeing that in your dream. Hence the advice of the scholars to one who is faced with this problem is to purify himself [do wudoo’] before sleeping, recite Aayat al-Kursiy, and stop thinking about sex and desire. If, after that, he sees something in his dream that he dislikes, he should not blame himself if no heedlessness occurred whilst he was awake. So pay attention to this, and bear it in mind.

Among the things stated by those who specialise in dream interpretation is that wet dreams like those that you see come from the Shaytaan, in order to make it easier for you to commit shameful deeds whilst awake. This is a serious matter to which you should pay attention, for the Shaytaan may also toy with the other party, and show her something similar to what you see in your dreams, for the same purpose. This is completely in accordance with those who say that the Shaytaan has something to do with wet dreams, and there is a great difference between the one who sees himself in his dream committing shameful deeds with his sister,  although he is chaste and decent in his dealings with her when he is awake, and the one who sees such dreams, and looks at her with desire when he is awake. The former is the one who is fearful and anxious about his dreams, and he is unlikely to do such a thing in reality, in contrast to the one who is tempted when he is awake, so the Shaytaan does not leave him alone when he is asleep, just as he did not leave him alone when he was awake.

Thirdly:

Some specialist doctors have stated that a person may do something when he is awake that will cause him to have a wet dream when he goes to sleep, although it should be noted that there is a difference of opinion among scholars concerning the cause of this phenomenon. That includes looking at women in unlawful ways, thinking about sex and desire, not urinating before going to sleep, and filling the stomach with food. We hope that you will avoid all of these things, and go to sleep in a state of purity and after reciting dhikr, so that you can rid yourself of these dreams which are causing you anxiety.

Fourthly:

The ultimate solution to your problem is to get married. If you are not able to do that, then you should fast, because fasting purifies the soul and leads to taqwa (mindfulness of Allah), which will help to prevent you from doing haraam things. Both of these are the recommendation of your Prophet Muhammad (blessings and peace of Allah be upon him) to young people like you.

It was narrated that ‘Abdullah ibn Mas‘ood said: We were with the Prophet (blessings and peace of Allah be upon him) and we could not afford to get married. The Messenger of Allah (blessings and peace of Allah be upon him) said to us: “O young men, whoever among you can afford it, let him get married, for it is more effective in lowering the gaze and guarding one’s chastity. And whoever cannot afford it should fast, for it will be a shield for him.”

Narrated by al-Bukhaari (4779) and Muslim (1400).

See the answer to question no. 9208, where we explained that wet dreams are something natural for which a person is not to be blamed. You can also refer to question no. 20229, which discusses means that will help you to lower your gaze; and question no. 20161, which discusses the solution to the problem of desire and how to control it. In the answer to question no. 39768, we discussed the health implications of the secret habit for the body, mind, face and memory, and we discuss ways of giving up this habit and avoiding falling into it.

Fifthly:

If you are suffering from this problem, and the Shaytaan is filling your mind with such thoughts about your sister, then you should take precautions to protect yourself. So you should avoid looking at her or being alone with her, for she is your sister. Tell her not to be careless in the way she dresses in front of you, for mahrams are not all the same when it comes to the ruling on showing adornment in front of them. A woman may show all of her adornment to her husband, but when it comes to other mahrams, such as her father, brother and the like, she should not show in front of them anything but that which ordinarily appears of her, such as the forearm, neck and so on. This is a basic ruling, regardless of problems that may arise, such as your problem.

But when a problem does arise, such as the problem that you are talking about, then precautions should be more stringent and covering should be more complete. But you should not draw her attention to anything that is crossing your mind. Rather you should try to guide her to do that through your parents, if possible, without giving any indication of what is going on in your mind or of what you see in your dreams.

The great scholar Ibn al-Qattaan (may Allah have mercy on him) said in an-Nazr fi Ahkaam an-Nazr (312):

Question: He may only see of her – meaning his mahram – her face, hands and feet, whether she is young or old. That is permissible, with no difference of scholarly opinion, because it is something that ordinarily appears (when at home), unless he looks at those parts of his mahram with desire. There is no doubt that this is haraam, and I think that there is no difference of scholarly opinion concerning that. Ibn ‘Abd al-Barr stated that it is prohibited, and he did not narrate any different view concerning it.

End quote.

Abu Bakr al-Hasani ash-Shaafa‘i (may Allah have mercy on him) said in Kifaayat al-Akhyaar (1/460:

It is haraam to look at mahrams with desire, and there is no scholarly difference of opinion concerning that. End quote.

And Allah knows best.