I haven't seen any animes with siscon trope so maybe that's why it's easier for me to have a charitable interpretation on Yuri's obsession to his sister that it isn't sexual but what Spy x Family has shown is that it's a series all about trauma and how it really changes people's lives.
First it was Twilight hating the sound of children crying which he eventually realizes that it's been triggering his childhood trauma. He hated so much what happened to him when he was young that he never want another kid to go through what he did. So that became his motivation for being a spy. And that motivation is what made him the greatest spy of Westalis.
In the case of Yuri, he would see his sister come home from her assassin job all bloody without an explanation (this is probably from Yor's early assassin days when she hasn't mastered cleaning up after serving her customers to completely wash away evidence). That's going to make a huge psychological impact on a kid that's still growing and developing.
And because Yor would bring home something he likes (possibly to get him away from questioning further why she's all bloody but also because as his sister she loves him dearly), he goes through these extreme swings of mood changes from fear and happiness without being properly consoled from the first emotion - it's just sorta stays buried and untreated inside him. I think that's what made him mentally unstable.
And on top of the trauma of having lost his parents when he was young, that sent him over the edge to wanna hold onto his only remaining relative.
He is overly clingy with his sister, but he has said that he wants her to find someone she can find happiness in because he became successful because of her:
He simply wants to make sure that the person she marries is someone who can do what he wants to do for her himself - protecting her.
Yuri probably wouldn’t be wishing that Loid and Yor get divorce and the Forger family out of the Briar’s lives if maybe Loid had introduced himself to Yuri as Yor’s suitor first and eventually ask for his permission to ask Yor’s hand in marriage.
Yuri maybe would have been reluctant to see his sister dating at first but he may eventually come to accept it. But Loid came in to his life already married to his sister without even asking for his blessing.
Since Yuri had only had Yor ever since from their tragic accident of losing their parents when they were young, Loid seemed like an intruder in his life. Yor being married for a year without him knowing seems to him like thinning out their bond since she is essentially starting a family of her own, without his knowledge at first.
With a different last name and different priorities.
My point here is that Yuri is reacting to Yor's marriage from the perspective of a mentally unwell person who hasn't healed from his childhood trauma. He's dealing with it with an unhealthy coping mechanism.
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