I don't say by this that the average LG/10/10 would really disapprove of the sole fact. Actually in mediaeval times incestuous intercourses sort of cemented family ties, especially in Dark Ages and far from the Pope. On some isolated island for example

I wouldn't. Even if I decided not to follow this path myself, I wouldn't blame anyone for doing so, my being LG. Well, in fact I couldn't condemn anyone for this even if charname and Imoen were biological half-siblings (no more than half - just the avatar impregnating two different women, I'm not a libertine). It doesn't really look outrageous to me in this particular case. Outrageous would be only the possibility of my own doing this... Well, but no point pondering the last variant - I've just recalled she's younger and from other source, so the woman were different. Still we can't be sure about the body Bhaal used...
As for the roleplay part, I don't quite agree this should be a slower progressing romance. It's a sister anyway. Some things are already obvious, the links between the two established. They're also closest friends possible and this really counts. In real life there are lot of things you do with non-romance friends of opposite sex that you don't quite do until until the romance is developing swiftly. For example the very emotional kiss on lips with Imoen sketched by Quitch is something classified as romance activity, but in fact goes with friends (XY-XY excl.). In Late Roman culture (esp. Eastern part) for example it had no less important meaning and almost the same conotation, but was also used when greeting members of opposite sex if emotional ties were (pretended/supposed to be) extremely strong. Emperors and their sisters often did this. But again, single persons and not within one sex.
I believe such tendencies and activities may well be employed in romance options for the mod. Starting from this, and then gradually deviating (well, this is kind of deviated sign of friendship anyway). The more PC & Imoen do, the more they allow themselves to do next time as borders have innate tendency to expand and so it works until it's clearly not pure brother to sister relationship. And they could even realise they have no qualms about this. It happened sometimes when people for example fell in love with twenty years younger/older step siblings they have hardly seen before. Here, because they technically are no siblings, the incest feeling is even weaker and could easily be overcome this way.