I don't like the story elements revealed in earlier spoilers that said that the imouto has been influenced by the third (and the final, albeit earlier) timeleaper and thus she is kinky like this.
My two reasons to be annoyed are the following:
① doesn't that make the third timeleaper, at the very least, a lolicon? — probably also somewhat of a borderline netori if that was done as a “take that!” to the protagonist,
② if there are only three time leapers, that does not explain (at all!) why Nakatsugawa called her senpai “the president” on the 21th page of the fourth chapter.
WTF.