Indeed, a strong 'cesty subtext with plausible deniability is in many ways even better than explicit incest. But if you can pull of the sex and keep everyone in character, than by all means do it. Either way, I'd love a strong emotional connection between the siblings as they do whatever it is they do--for Valentine and Peter, take over the world (and please retain as much of that element as you can!)--for Annie and Hallie, grow up and change and learn life lessons (however you think they would)--for Laz and Lor, well, have sex, because they're cloned from a horny old bastard.
Oh, and if you're going to write Carmen/Player? Then you can pretty much ignore the above. And also I love you.
In my profile information I say that the most important thing is for you to write a good story and to stay true to your muse regardless of whether I end up liking it, and I stand by that (if I knew exactly what I wanted, I'd write it myself), but I did make these requests with a little more eye to my kinks than usual. If you want a better idea of what I like, feel free to poke around a little bit--my fic is linked in the pretty sidebar on the left if you're viewing my journal in my format (I tend to write what I'd like to read), plus there's the tags and just skimming (memories are in disarray, so I'd recommend you not try to use them) and so on--and have fun!
Yours in La Mancha,
Episkopos Reverend Alixtii O'Krul V, TRL
Church of St. Jesu the Heretic, Discordian