Who am I to sneer at a gift from the google gods? If writing about sister sex will boost my blog ranking, then what the heck, I’ll write about sister sex. Or rather, write about writing sister sex.
We have such a strong taboo against incest that if you want to write about sibling sex in a mainstream novel, the incest must be motivated. In other words, there has to be a strong reason for it. Perhaps the children were shut away in an attic most of their lives and had only themselves to rely on. (If I remember correctly, this was the premise of V.C. Andrews’ Flowers in the Attic.) Perhaps two sisters were molested by their father, and the only tender love they knew was the love they offered each other. Perhaps the parents saw nothing wrong with sibling sex, perhaps even encouraged it by having a brother and sister share a bed. (Of course, this complicates matters in that you have to show the parents’ actions as being motivated. Why would they think this was an acceptable arrangement?)
The point is, if you want people to accept the incest, you have to give them more of a reason for siblings to have sex than simply because they wanted it. People want lots of things that are not acceptable, but that does not make the thing acceptable to lots of people. As always, with writing, you need to figure out what your motive is for writing the scene before you can figure out your characters’ motivations. Are you trying to prove that sibling love is acceptable? That it’s inevitable under certain circumstances? That love is love wherever you find it? That you have the hots for your sister or brother and want society’s okay? Whatever you want to prove, you then have to write the scene with this objective in mind.
Once you get past the motivation and emotion leading up to the sibling sex scene, writing a sex scene with a sister is the same as any other sex scene. You show the two together, you show them connecting both emotionally and physically, and at the end you show how their interaction affected them.
The emotion does not have to be love on both sides. Nor does it have to be a single emotion. There could be love coupled with revulsion, love with fear, hate with sorrow. The emotion and how it affects the couple, how it changed them or defined them is what makes the scene compelling, and the more contradictory the emotions, the better.