I have been fantasizing about my mother and sister since I was 17. Now I'm 25 and I still turned-on when I have such feelings. It is really the only thing that makes me happy. I'm not willing to act upon my feelings. These are just thoughts in my mind that really makes me relaxed while masturbating. They are just fantasies. I'm not harming anybody. Is it haram?
Unsurprisingly, dwelling on sexual fantasies about one's mahrams is not encouraged:
You have to repel this deviant feeling and keep away from all the reasons that may lead to it or help in it, even if it is looking (at her).
Feeling attracted towards his mother, IslamWeb
However, whether incestuous sexual fantasies themselves are haram, it might be covered by ahadith such as:
Verily the Great and Mighty Allah forgave my people the evil promptings arising in their minds, but they neither talked about them nor acted upon them.
Sahih Muslim 127 b
Islam Q&A discusses the scholarly opinions of whether this should apply in the context of (non-incestuous) fantasies during spousal sex:
It seems that the correct view is the view that such fantasies are makrooh, even if we do not say that they are haraam.
On top of this:
Masturbation is considered haram (with the caveat that it's better than committing fornication, sodomy, etc.). See Is masturbation permissible for a man during an extended period of unmarried life?
Incest is considered haram. See Is having sexual relations with a mahram permissible?
The act of having erotic feelings about your mother, sister or any other mahram is not haram, but very shameful indeed. So, shameful that Islam didn't even bother to discuss them.
Therefore, as long as you don't rape your mother or sister, you are not committing any sin. Because you will not be judged by any bad thoughts that crosses your mind. As explained in the following Hadiths
Narrated from Ibn ‘Abbaas
The Prophet (PBUH) said: “Allah decreed good deeds and bad deeds, then He explained that. Whoever thinks of doing a good deed then does not do it, Allah will write it down as one complete good deed. If he thinks of doing a good deed and then does it, Allah [may He be glorified and exalted] will write it down between ten and seven hundred fold, or many more. If he thinks of doing a bad deed then he does not do it, Allah will write it down as one complete good deed, and if he thinks of it then does it, Allah will write it down as one bad deed.”
Al-Bukhaari (6491) and Muslim (131)
In another Hadith:
Narrated from Abu Hurayrah
The Prophet (PBUH) said: “Allah, will forgive my ummah for whatever crosses their minds so long as they do not act upon it.”
Al-Bukhaari (5269) and Muslim (127)
Now as far as the act of doing masturbation is concerned... Some scholars have derived that the act of doing masturbation is prohibited. Whether you do it with your mother in mind or someone else's mother. I understand that's not your question to inquire if masturbation is allowed. But since this website is all about knowing Islam, for the sake of other readers I am putting some light on that as well... So, instead of doing masturbation, it is more preferred that you get married and have sex with your wife. Or you can also buy a concubine for yourself and have your lust for sex quenched on her. I hope that way you will stop having intimate thoughts about your mother and sister.
Since I have been doing this for many years, it's unlikely I will ever change this habit. I hope god will lead me to the right path. If these dirty thoughts are the only things preventing me from committing incest or zinah, then I will keep on them and have whatever thoughts I would like as long as I don't commit zinah.
deliberate fantasies are haraam for no matter whom let alone the mehram. brothers Quran and Hadith will not contain every tiny details of life. This is where fuqaha come handy. They decode the hints from the holy book, sayings or prophet (pbuh) and the life of him and his companions and sometimes, if needed, apply their own logic by Allah's authority. I am not a scholar and i neither came across any content about incest fantasies but i was googling up whether fantasies are haram in general. I read an article where i found most clerics are of the thought that it is haraam to dwell into fantasies while its is okay if they come involuntarily to you and you wipe them off. The very next article in the search result was this. I was shaken that the brother said that its not haram to fantasize about own mother. If it is haram to think about anyone how on this earth can this be okay to think about your mehrams. Allah knows best here is the link to that article https://islamqa.info/en/84066