Wtf is this sh*t? Plot didn't make too much sense before, but this is too much; the trope itself is already stupid enough, but here is brought to the extreme and beyond.
If FMC remains friend with her after this is just garbage
If you're gonna do this shit then what was the point of trying to make Onee-san look like the main love interest? We could have kept fun girls like the art girl or the hot landlady if the plot was going to contrive scenes like this.