I ran searches for anime featuring a school that trains students to fight monsters in dungeon-like areas, involving siblings (especially sisters) and one possibly named “Stella,” but I couldn’t turn up any series that matches all those specific plot points closely.
One partial hit: Chivalry of a Failed Knight (Rakudai Kishi no Cavalry) is set in an academy where students duel. The heroine is named Stella (Stella Vermillion), but here she’s a skilled duelist—not a sister—and there aren’t dungeon missions or memory wipe plotlines  .
It seems the anime you’re remembering might be pretty obscure or perhaps even a one-off OVA or light novel adaptation. The “Stella” name could be misremembered or coincidental, and the unique elements like fortune-cookie scenes, a school cover-up, memory wiping, and a sister turning into a monster inside a dungeon together make it a very distinctive storyline.
To help further, could you share anything else you recall? Even small details could spark a memory—like any visual cues (character outfits, emblem, fantasy elements), where/when you watched it (TV, streaming, dubbed/subbed), approximate era or animation style, or any other character names or specific lines. These clues can be really valuable for pinpointing something obscure.
