This trope has been abused by Anime lately to the point you can't even tell why it exists anymore.
In my humble opinion, this trope shouldn't be evoked just for the sake of tasting the forbidden fruit but rather to show the character's mistrust in people who aren't blood-related. Controversial as it is, there are a lot of historical acts of it in real life.
Most of though has nothing to do with 'love'. Some eastern clans kept their 'bloodline purity' through marrying off siblings while others had their own political reasons to do so. Either way, there are rarely those who marry their sibling by choice.
But those who do are an example of what happens when you confuse 'familial love with romantic love' or just because the fact they are blood-related isn't found out in tragedies. Though most realistic people in these circumstances act appropriately.
Either they start their relationship from scratch or deal with the consequences if they have already started a physical relationship. Either way, nothing in real-life is as optimistic as this working out. Not unless there's a old governmental law in place.
I emphasise on 'ancient customs' since even the Egyptians found this normal. I could list the number of reasons a normal and rational individual would choose to curb-stomp any less-than-innocent feelings for their siblings as soon as they feel it.
However, the 'familial love turned romantic' is a subject much more interesting to me than a forced marriage or just not knowing the other party is blood related. That's because it takes certain qualities in the main character to evoke this trope.
Firstly, they must have a concrete cynical mindset towards others and overvalue their relationship with this sibling in particular. Whether the siblings is older or younger, girl or guy, they must have this sibling at the centre of their universe innocently.
After all, no good 'Sibling Complex' is anything like in Anime... Even if there was a person who got attracted to his sibling physically, their common sense would make them crush those thoughts.
The difference between those who confuse familial love with romantic love and a normal person is either how they obsessively put this sibling on a pedestal as if no one can beat them, or how they seek true 'understanding' in their life partner.
Most siblings have lived with each other from childhood and are openly honest with each other to a fault. There are, if not full aware of each other's strengths and weaknesses, are still the kind of people who would never judge their sibling outwardly. All you need in a life partner is right there if twisted the right way, the only thing left is a motive. Why would the character pursue a sibling?
They can either be immature like a teen, or it could be circumstances that make them act. What I am trying to say that a 'Sibling Complex' works backwards compared to a regular relationship.
You've already known your sibling for most of your life, you are already certain about the things you can and cannot do, now all that's needed is the incident that makes them turn this 'romantic'.
Saying that there are a lot of ways for you to write about their familial love turning romantic isn't true at all. After all, why wouldn't the character be content on their relationship with their sibling being innocent? You HAVE to give a good reason for this.
If not, you risk turning your character into a guy who only thinks with his lower half. What good is there for character of such nature? The best way to avoid this is making circumstances force his hand.
I don't mean societal wise like forcing them to hook up because of traditions. You can do that but you'll miss the chance of adding social development which affected the character's heart and mind.
Perhaps the character was bullied all their life and only found solace in their sibling, perhaps there was once incident that deeply rooted into the character's heart and made their thoughts messy.
In the end of the day, you must be able to show an emotional reason for them to evoke this trope.
Moving on:
The second quality the character needs is some self-awareness, they need to understand the consequences of their actions and just how their actions are seen by others. The self-conscious protagonist tends to make their sibling suspicious as their previously innocent demeanour gets covered by a mask they put up. The sibling will then become inquisitive and find out the truth.
It's just depraved to not add the character struggling inwardly as even this sibling of theirs might not respond to their 'romantic affection' in a positive way. It would be tragic for their relationship to break due to him acting on his toxic thoughts.
In the rare case that the sibling returns the character's romantic affection, they are placed in a worse state than Romeo and Juliet. They are bound by blood and know of the forbidden fruit, yet they they still desire it regardless of the world around them. Though love between siblings is a sin, can you create circumstances in your story to justify it?
You probably CAN do it. It won't come off as a bad piece of work that gives out the wrong messages if you are self-aware about this trope, admit that this trope cannot be evoked by a rational and logical main character, and make it more than about the romantic aspect of 'sibling love'. Though you can cater to a niche audience, a regular romance would be much more of a intriguing option if there's nothing else going for the story other than this.
That's all I got for you. See you next time someone makes a request or I get inspired. Peace out!
