Shoo, imouto. Your fetish doesn't belong in this world. It's all teacher-student in this reality. I'd approve of the wincest if it started the manga like that, but it did not. The imouto should back off and remain a supporting character. He didn't leap time to get his teacher NTR'd by his cute imouto after all. At least she didn't write something on his forehead while he was sleeping, though at this point it's the obvious expectation based on how many times I've seen that joke.