The plot goes like this. The "copy" is actually the real one and the "tsundere" one is a copy. The copy want to help the real one so they both pretend.
And then it suddenly got dark, real imouto revealed have short lifespan and in the end, when the real one died, the copy become "real" imouto and said that the "copy" just disappear.
All because both imoutos don't want onii-chan become sad.
And then again, it is just a one-shot and i just spout some nonsense