Oh, of course I'm going to search for it now.
---Searching---
Ick. I've heard of that kind of thing before, just more EXTREME S&M. Good thing there weren't any examples.
And yes, guro would much more of a concern than something that has overall positive representation of some taboo (at least as far as I can tell). Really the idea that it would be brother and sister or sister and sister and dude or whatever the case may be is most often used as shock value or for titulation of it being some kind of taboo-breaking thing. But as far as I can tell, Kana seems to be using it for artistic/philisophical/moralistic value, in the debate as to how right it is for you to grant your dying sister's wishes (I think that we can ignore the added on, "Psyche, she wasn't really related to you after all,").
Now if the question is about between incest and the idea of having something go on with you AND twin sisters, it's a little different in the first and latter case. Incest would mean you are related to said person(s) you are engaging in sexual acts with, and the twin sister fantasy is the idea of being with two girls, and I'd think that that case would be more readily acceptable than the former, just because of the idea that it isn't centered around them with each other, more as them with you. Granted, it is most likely that they will be getting physical with each other too, but that's just the nature of group sex (i.e.- more than two people, since that could easily get misinterpreted, I wanted to clarify there). As for repurcussions, I can't really say, because in real life that is a serious thing to consider, whereas in any ero-(media), it would be either ignored or purposefully used as far as I'd guess. It also seems as though they deal with it in a less-than-realistic way from the standpoint of an average guess as to how it would happen. I agree with FT in that frequently if it goes beyond child sexual exploration, that it would often be exploitative (my friend knows this fact quite personally, but that case was even a little more different), but from the occasional case of purposeful and happy incest, I'd say that even then it would be a serious strain on at least one party who isn't sure so much as the other. And that is because every time something about taboos was brought up, it would be maginfied in their perception, and create a great deal of stress, rather than be this somewhat-stressful-at-the-moment kind of thing. The parties engaging in the acts would be worried, and then they'd either have to continue to go to more and more extremes to keep it secret, or they'd have to move to France.
Where was I going with this...?
Okay, so that switching it off to guy and girl having sex vs. incest thing is also a personal perception thing. You are seeing it as something not necessarily good, but not at the same time bad. Now, if it was something like rape or exploitation, then that is an insinctual "bad" case, which is what you would then view it as (I'm assuming). But if it is so demanding in someone's mind about it being incest and therefore wrong, they aren't getting the message that it isn't supposed to be easy for those kinds of things to happen (look at the series Boku wa Imouto ni Koi wo Suru, that's some damned strain there). Frequently, I'd say, it tends to focus more on the acts of hiding at incest, rather than the strain associated with it, based on the outside world's perceptions.
Okay, I realize I have thrown myself off the cliff of topic somewhere in there, so I'll stop there and just ask me any for any of my other opinions if you want, I'll try to be more direct if I can.