Question about my fantasy story?

I'm sorry but I don't see a question here. Are you asking for remarks about your love-story? Whatever; just get busy writing and get the story finished. The Duke, who wouldn't be a Duke born of a common prostitute, and the Queen may not know they are related. The Duke wouldn't be able to prove his claim to being the late king's father anyway. That they are related and lovers is not even unusual for European nobility of the middle ages; they often inbred to keep their line 'true', though rarely did brothers and sisters get together. The Romans, if I recall, did that, as did the Egyptians.