monkey123, sorry for the atrociously late response...
I don't doubt what you read, but where is it cited he is an Incubus. Where is it cited his original mother (of this world) isn't something other than human (or his father). And where is it cited that being in her womb changed his genetics?
If he was altered magically he isn't related to her... Not in the normal sense (not if you don't want to state all entities that create some other entity are therefore directly related to them ...) You would basically be stating that if Vampire A turns Human B into a vampire to live eternally with them their relationship would be incestuous...
Yes, I know it looks like I am making this more complicated than it needs to be, but let me flip this around to make it as simple as possible: if it isn't explicitly stated or shown it isn't. Traps are the shown gender until proven otherwise (or everything is a trap until proven otherwise, if you will). With fiction that is my philosophy (I use traps as an example because it is simple and simple to prove one way or another) and I am (mostly) satisfied with it...
(Which is why I'm so aggravated with fiction that SHOWS people in the buildings those monsters are bumping into... Such needless death!)