Personally I'd say no to both - IMHO two people sharing similar DNA is far less important than their father being the same person. Incidentally, the British government has recently introduced a law which makes it illegal for even step siblings to have sex (though I'm not sure what happens if a married couple's respective divorced/widowed parents were to meet, fall in love, and then get married themselves )...
You know, my father actually works with a man who is married to his step-sister in this way. First, his coworker met his wife-to-be and became married, and through their relationship, the couple's widdowed and divorced parents met and later were married. It's kind of off topic, but I like the story.
Since Imoen and male protagonist a) aren't genetically brother and sister and b) weren't raised as brother and sister, I wouldn't go so far as to call their relationship, should they have one, incestuous. Personally, however, if I were playing a male character, I wouldn't want to romance Imoen. I ditched her in BGI for just about every other character and tend to become annoyed by a lot of what she says in BGII. I know, I'm terrible.
And for the pedophilliac argument: 15 really isn't all that young. I was dating Seniors aged 18/19 when I was 15. Then again, everyone thought I was a Junior at the time because I wasn't covered in pimples and didn't act like an idiot. It's a personal maturity thing. Since I've never played the mod, I can't really say anything about it. I'm incredibly wary of downloading new NPCs . . .
It would sit a little better if she were even a year older, however. Which is funny, because the difference is only a matter of months.