Harleycest Dynamics
so... you know the Harley twins? yeah, Tes and Sébastien. they're both triple scorpios in love. that's a recipe for a disaster. a hot mess, even. take the most dramatic romance plot you've heard of and amp it up by ten times.
these two have tried to kill each other several times. why? because they love each other. true love, passion, and the desire for revenge will lead you to do irrational things... and sometimes those irrational things are dangerous. i'm gonna talk about how fucked up they are for one another and how that makes them perfect.
both their psychologies are incredibly complicated and i'm gonna ramble about it whether anyone reads this or not.

🌙 Canon Timeline - Sébastien Dominant
yeah, you know. the one with Sébastien being Archbishop of the Lunar Church and eliminating La Populace, and Tes being the enigmatic detective.

Sébastien
Dear brother... My only twin; my mirror image. Why do you reject me?

Tes
Because I see our mother in your eyes. And frankly, it horrifies me.
Sébastien rules with an iron fist and supreme power, but everyone who works under him is absolutely terrified of being taken out when (not if) he finally explodes.
Tes spends the majority of the first arc avoiding Sébastien, but not because he hates him or anything. it's because he can't handle losing another person he loves. he can't bear to see Sébastien lose himself to his own rage and desire for revenge for their (very abusive) mother.
everyone who Tes has gotten severely attached to has died or turned away from him, whether it be due to his acts of cowardice or circumstances he couldn't control.
that's the thing about Tes, he desires control. in fact, he has so much of it that you'll never see him flinch or choke emotionally anymore. remember, this man has literally ditched any semblance of an identity as well as his balls to become a faceless, nameless shadow.
ironically, this makes Tes more of a dominant character than Sébastien. but as much as Tes likes to act like he's invincible because of his lack of emotional attachment, he isn't. in fact, he's running from the last person he truly loves because he doesn't want to see him die.
Sébastien can't stand this and basically does anything he can to take jabs at his older twin for ignoring him. because just like his own brother Tes, Sébastien can't let go of him.
not only did Sébastien pass laws to ensure that Tes would be caught and sent to his dungeon, he also wanted to "purify" him of his shadow "corruption"... how? by execution through tossing him into a pit of lunar fire to melt him into a horrific glass statue, of course.
by this point, Sébastien has fully accepted the Lunar Church's religious ideals due as a coping method for his grief. he's not really there in the head anymore... but instead of being a follower, he became god itself.

Sébastien
Brother... I'm sorry. This is the only way to save your soul. I can't bear the thought of you being eternally condemned to damnation.
I forgive you for straying from the path, but the Moon will not. Therefore, I must make this sacrifice.

Tes
Save me?! You know you're about to throw me into a pit of blue fire, right?!
obviously this doesn't happen because Tes escapes and proceeds to save Sébastien from his own undoing. there is an alternate ending where Sébastien does execute him and Tes' glass statue can be found within Sébastien's room because he failed to escape.
it's eldritch - Tes' statue is an abomination of shadowy manifestations as they were torn out of him in a desperate attempt to avoid the scorching lunar fire.
his face is contorted into a expression of agony and sadness. but the good thing is, he would never live to see Sébastien be consumed by his own anger. one can see the bones in the statue.
it doesn't stop Sébastien from being creepy with his statue, though... constantly polishing his brother's glassed corpse, whispering his adoration to it during the night, and crying his sorrows and regrets to it.
the story will end in Sébastien holding the decapitated head of his brother's statue to his chest, crying to it and begging it for forgiveness before he dies from his injuries.
obviously... Sébastien's wife, Selena (who he married for his insidious motives that don't include loving her), is very disturbed by this. i don't think she's very happy about being cucked by a glass statue.
it's alright though, as she comes back in arc two for revenge... and redemption.
tl;dr: Sébastien is extremely unhinged, lacks control over it, and Tes is terrified of it. well, and also aroused by it.
the only thing that turns him off is that Sébastien reminds him of the very same way their mother acted.
✂️ Alternate Timeline - Tes Dominant
so this is the one where Tes and Sébastien swap their roles. instead of Sébastien becoming Archbishop to avenge his mother, friends, and family, Tes becomes Lord, assassinating the previous leader of La Populace and eliminating the Lunar Church using their ranks. Sébastien is a sexy detective with a secretary look in this alternate universe.

Tes
My Moonflower... Please don't run. I can keep you safe. Why can't you see that?

Sébastien
Brother... this isn't what father would want, our mother-

Tes
That wretch KILLED him, Sébastien. Don't you dare speak of her!
wanna know something funny? Tes is much more terrifying than Sébastien in a position of power.
Tes and Sébastien's goals are similar in both universes in that they are both motivated by the will to change the world around them so that no one would hurt the same way they did. while Sébastien avenges their mother, Tes avenges their father.
since La Populace is a mafia and not a church organization, Tes rules through psychological fear and terrorism. under his rule, there really isn't a lot of rules. you look out for yourself, no one is truly your friend.
in order to protect himself, he makes himself scarce... so scarce that he refuses to be acknowledged by any name but his title, (My) Lord. not even Tes. i'll still refer to him as Tes. call him Jonathan if you want him to strangle you with a shadowy clawed hand and make you disappear without a trace.
in fact, he's looking to capture and contain everyone who knew him as Jonathan. he can't kill them - he's unable to lose another person he loves... so imprisoning them forever and brainwashing them to be his loyal servants is the next best thing.

Sébastien
Why...? Why are you doing this?

Tes
My Lunar Blossom, I told you. I'm keeping you safe.
No one shall pluck your delicate petals but me.
Tes' favorite person, however, is Sébastien... who he does capture and intends to marry.
now here's the thing - Tes is much more in control than Sébastien is, in any world.
instead of Sébastien trying to forcibly burn Tes alive in a pit of lunar fire, Tes literally mindbreaks Sébastien using shadow magic and hypnosis.
so you have Sébastien with his makeup running for half of the day because he can't stop crying shortly after reapplying it. Tes knows the best ways to get into his younger twin's head.
there is one point where Sébastien tries to kill Tes with one of his hoarded mementos, their mother's tailoring scissors... which is ironically Tes' basis for his shadow weapon's form - a giant pair of shadow scissors.
however, this fails. Tes' body is far too corrupted by shadow magic to die from a stab wound. Sébastien reels back and collapses to the ground, terrified for what punishment would come for his attempted assassination... but Tes simply laughs, crouches down and caresses his brother's crying face with his clawed hands, saying he can't get rid of his own shadow.
Sébastien does escape and put a stop to Tes' tyranny... but like canon, there is an alternate ending where he doesn't.
in this alternate ending, Sébastien is thoroughly brainwashed and mindbroken over months and years. Tes uses a combination of both his hypnosis and classic mind games to break him down.
Sébastien becomes Tes' adoring, broken wife. there is no soul left in him or any more tears to cry. it's really a classic case of Stockholm, but ten times worse.
tl;dr: Tes is a possessive, overly controlling monster. Sébastien might be into that.
but hey, on the plus side: the canon ending is that Tes snaps out of it. their relationship is healthier in the good ending.
Final Thoughts
so basically, their relationship is far from perfect. in fact, no one's relationships will ever be perfect, no matter how close you are to someone or how long you've known them there will be times that they will piss you off. you are not perfect, neither is your partner.
conflict isn't toxicity. the idea that relationships must be pure, healthy, and perfect is toxic itself. it's something you overcome with your partner in order to create a loving, mutual relationship. there will always be challenges that you must overcome within a relationship. hiding from the issue will never solve anything.
Tes and Sébastien are just an extreme case, and obviously incest is not something you should be encouraging or endorsing in real life. but hey, who cares. it's hot in fiction.
they are always married in both "good" endings. the key is that they work through their problems together, even if it hurts. you both have to be willing to cooperate and put in the effort, otherwise it'll fall apart.

because trust me, it's worth it in the end.
and just so you know that i know what i'm talking about: my soulmate of thirteen years and going is a scorpio. my best friend and partner is a scorpio. my mom is a scorpio. you can only imagine how turbulent my life has been.
so yeah. i know everything about intense love and overcoming conflicts most people nowadays would give up at. there's one thing that scorpios are good at, and it's being as dramatically extreme as possible. that includes their relationships.