Isekai de Ane ni Namae wo Ubawaremashita

Through a handheld mirror connected to another world, college student Ichika shares a friendship with a boy named Cecil—one that only her older sister, Karen, knows about. When Karen and the mirror disappear one day, Ichika is devastated. But grief turns to shock a year later when she, too, is sent to the world on the other side, only to find her lost sister and a grown-up Cecil! Except...why are her sister's first words to her "I've missed you, Karen"? And why is everyone else calling her "Ichika"?! In a world where her name and her past have been stolen, Ichika must find a way to forge her own destiny!

(Source: Yen Press)