@zdarkbrz_816
Why what? For once, this is actually a very plausable situation. Nothing is too extremly out of the ordinary. A sister stopping by her older sister's appartment is normal. And since she is in highschool, her mother giving her a spare key is also very likely since she doesnt want her to just be sitting outside the door if her older sister isn't home at the moment. Not calling ahead, because it is a suprise visit.
Smelling her bed and being able to tell another person was there could be a stretch, but still possible if they had a distinctive scent, sweated in it a bit recently, or wore something like perfume, cologne, or a scented deodorant.
If you are asking why this situation was written in the first place, idk. Maybe the author felt the story was getting somewhat repetitive or too bland? But I do like how they did it with the sister and not someone like her boss or strict neighbor or something. The sister, at least for now, does not seem like too much of a siscon where she gets annoying. As for why she is watching, stuck in the bedroom and missed her chance to leave and make her presence known.
As for why she is getting turned on... maybe its her kink to secretly watch other people and she just has yet to realize it. It is a fairly common response though. Watching 2 people getting intimate can put you also in the mood even if you were not intending to.