@morose I think Philene is more suspicious than these two plots, the MC didn't know about the hole and she “jumped to him” from nowhere after he left the mansion. (like, she was waiting for him, playing the role of a reckless little girl.)
It’s just a theory, we have a dream of his past about his childhood friend, but I think it is a 4-wall indice for us. Showing us how the sister will act and play the "lover" route, nothing can beat childhood friends that flirt every time after all. Maybe when other girls we gain an interest in the MC we get to see her true color.
Of course, it’s possible none of them are the sister, and we meet her again in the school arc. This Manga it's interesting in that sense, she can come from everywhere and put a mask. For now, I put my bet on Philene.