I prefer brother+sister or sister+sister. And it doesn't matter if she is declared blood-related or stepsister.
Brother and sister or sister and sister have a more equal level relationship, when they are at the same age and especially in comparison to the other possible relationship combinations. Brother and sister are on equal mental footing. They can (or often are) more close than best friends could ever be. They have a natural bond and share many memories together, grew up together. That special bond and intimacity (or familiarity
) makes the friendly relationship (and especially potential romantic and perhaps later even sexual relationship) way more intense. It makes it easier for the writer to create those intense and intimate moments of a bond if you use an possible incest theme.
So to me, the stuff I like about brother+sister incest or incest in general is that. The intimacity, the strong bond and emotional attachment. I don't care about the blood-related stuff. But the thing is, with an incest theme, crafting such an atmosphere works the best (or easier if you want to phrase it that way).
And another thing, in fiction you can – thanks to suspension of disbelief (and because it's a hot sister but not your sister) – mentally hide the things that in reality it would bug you, in reality would be weird and yuck to you. But within a fictional world it can be hot, because of all that what I have mentioned above.
Because of all that. In an adult game where plot and story means nothing, where it's solely(!) about a quick fap, there the whole incest theme wouldn't work anymore, at least to me. Because in a story-less incest game there would be only left the yuck parts of an incest theme, but not the bond/intimacy part of it.