So is this some sort of immaculate conception akin to Rabbi Yeshua?
Afaik, this is like Zeus going around impregnating random women and there has been an exchange of bodily fluids involved. I.e. Imoen is your half-sister.
They should have made her in to a lover,a someone to keep you rooted and a good reason to not just become a murderhobo god.
I'm sure you realize people care about their sisters as well, not only lovers (let's be real, sexual conquests).
She wasn't supposed to be your sister, not until late into the development of Baldur's Gate II. That's one.
Second, children born out of gods + mortals in Greek Mythology would often be... unusual in physical appearance. A lot of the monsters were also birthed by liasions with gods, keep in mind.
Third, Bhaalspawn always, always, inherit the race of their mother. Not a single one of them was in any way, shape or form altered by their father, which is the biggest piece of evidence that what he "donated" had little to do with genetics, and a lot with spiritual corruption. Heracles could not just take out the Zeus part of him and give it to someone else - Bhaalspawn can.