The story begins with Takumi's secret fantasies about Saki. Their relationship shifts after a chance encounter on a train, eventually leading to the discovery that they share similar secret desires.
Reviewers have noted a distinct art style that some describe as "chunky" or slightly dated compared to modern productions. aneki... my sweet elder sister the animation
Aneki … My Sweet Elder Sister: The Animation (2007) - aniSearch.com The story begins with Takumi's secret fantasies about Saki