I want to write a fantasy epic which is loosely based on historical facts. The hero is a Prince and next in line to ascend to the throne but the complication is that for years he has been inseparable from his mistress. They spend quality time together, share personal thoughts and feelings with each other, etc. He is more observant and reserved while she is more sociable but bratty at times. However, both are kind and compassionate to each other. His parents tolerate this despite the partner in question (I’ll get to that in a moment), but they want him to wed someone else from another noble family. Seems like an ideal fairy tale romance but (here is the controversial part) his mistress is also his older sister.
It is not intended to be like Game of Thrones where the sibling have an abusive or coercive incestuous affair nor one where they intend to breed with one another. Rather it is a sexual relationship in which the siblings are consenting adults (M19 and F22 when it starts, but the story takes place a few years after they’ve started seeing each other) find each other physically attractive and frequently have casual sex for comfort and pleasure. I want to convey that do genuinely care and respect each other as siblings but are also physically drawn to each other as…well since there isn’t another way to describe it…as siblings with benefits.
So how do I convey that the hero is a good guy despite having his own sister as his active sexual partner and vice versa?
How do I make it so the emphasis is more on how much they care for each other while also conveying that they are together purely for both emotional comfort and for physical pleasure, not necessarily true love?