I was wondering if any group reactions to anything that Imoen says is planned, or if the male PC starts flirting with her, will they comment on it?
I know it's not a romance mod, but thinking from what your saying, anything is possible.
Viconia, I don't see her having any problem with it, at times during her conversation about her brother, she showed that she deeply cared about him. (Plus I would not put anything pass a Drow)
Aerie on the other hand I would see having a problem with the PC flirting with his half-sister.
Take care and hope I don't start a whole mess with this question.

All the NPCs will comment at appropriate junctures. As ever, Return to Windspear will act as a guage and test of my
NPC writing. I know some better than others.
As you say, from the player end, almost anything will be possible. Obviously I'm only going to be able to cover wide areas, like flirt, friendship, bullying, etc. I can't be too specific, we'd be here till the end of time.
Viconia wouldn't have a problem with it because she doesn't believe a true genetic bond exists between you and Imoen, while Aerie hasn't taken any written stance on the issue. I would imagine she would be against it. I imagine most of them would be, including Imoen.