they ARE twins tho. i don't remember what happened to her brother tho...run away maybe and then she started acting as him LOVE
She never grew up with a twin and it was an unknown boy's name in the family register tho, so she thought it was her father's doing, readying it for her when he dies..
I'm suspecting the prince actually knows she's a woman and there are no twins in the Artur family (● ̄(エ) ̄●)