Your doubt is justified. Are Incest Stories real or fake? About 90 % of Incest Stories are based on Fantasies. It is obvious that if you go to a Site that has Incest Stories then all those Stories will be of that genre. This is not Reality but just catering to what the Reader whats to imagine. Even on AA you may come across many questions on Incest, but a large number are the fertile imagination of Members here. This is not to say that it does not happen. It does, but not in the proportion that it is made to be.
In ancient Egyptian royalty, around the time of Caesar, marriages between brother and sister existed and even encouraged.
Incest happens, because of the close proximity between Family Members. Joint Family where Uncles live with Nieces and small accommodations where families live in one room all lend to occurrence of Incest. You often read about Fathers raping daughters in newspapers and this becomes news only when the girl child complains to an Elder.
Freud spoke about childhood sexuality. The memories of childhood linger and are carried into adulthood. So games like doctor-doctor, and hide and seek in dark corners and the occasional groping leave an impact on fresh innocent minds. There is also a hero figure in the girl child’s mind and her brother is often that hero, whom she worships and wants to please. Often parents leave elder brothers at home to take care of their little Sisters and this is potentially dangerous. Sexual behavior is likely to occur, especially in childhood, when a young person is being supervised by an older male. The older male does not have to be someone who is obviously disturbed or perverted. Some situations invite sexual acting out—for young men who might otherwise never think of committing such acts.
Raj, now lets look at your problem. Your Sister has in no way encouraged you or there has not been any past incide3nt with her that has led you to believe that she may be having the same thoughts as you have.
You need to strongly curb your instincts. I am sure your Sister will react strongly if you suggest anything. She may also complain to your wife and this would lead to an awkward situation for your Family Life.
Fantasies are fine. But let them be only fantasies.