I really wanna see the explanation of WHY he's a Calamity and ostracized like that instead of working with him, his sister can literally instantly kill people with her ability and it's fine...
I imagine a part of it's the fact he's male.
Women sit at the top of the social and power hierarchy of their society because of the disparity in average power of abilities between the sexes. Men are, thusly, heavily discriminated against and treated as second-class citizens.
If it came out a boy of all people was the
first S-Class ability wielder and that it worked (as far as we know) by sexually dominating a woman and bringing her to orgasm to
sever her soul from her body, letting him then control her or even puppet her body with another soul?
That's a terrifying threat to power structures everywhere. They would have had to strangle him in the crib or something, because the moment he gets any wind of what he could do, he could get it in his head to "take back power" for men everywhere.
It really might just come down to "we like being in charge and he threatens that very concept".
Now, I would expect they would just kill him already and be done with it. But there's a chance that he can't even be killed, given he deals in ghosts/souls already. If they even suspect that,
or if they already tried and failed and he just doesn't remember (perhaps something to do with the father dying, thus sparking his stepsister's wrath toward him), simply letting him think he's trash-tier and hated and keep him oppressed and without friends or help would be a way to contain him.