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UP DIS! LOOKS INTERESTING #－.－)
This is the only similar thing i could find, it doesn't match the description exactly but has very very similar things going on at some point https://www.mangago.zone/read-manga/tengoku_daimaky/
This isn't it but thank you for trying!
this sounds interesting lmk if you find it!!
A girl and her brother want to learn magic. They meet a boy who takes them to a strange mansion owned by a scientist. The mansion is full of weird experiments. The brother has to stay outside while the girl goes in with the boy.
Inside, something goes wrong, and the girl ends up dying. But the scientist brings her back to life—except she’s now possessed by him. The boy fights to save her and eventually succeeds.
Later, the boy meets his big-chested friend, and they chat. The girl feels jealous, though, watching them. In the next chapter, the big-chested friend dies mysteriously, and the three of them have to figure out what happened.