Tensei Shimai No Otoge Kouryaku - Ane, Chanto Flag Tateyou Yo! summary:

Himari is the plain sibling, whereas Yuri, her older sister, is the perfect superhuman. Yuri, Himari's pride and joy as well as the source of her inferiority complex, suddenly disappears one day. A year later, Himari gets a phone call from an unknown number, and it is Yuri who has reincarnated in an Otome game! Himari is supposed to help Yuri navigate her way to the ending of the game so that she can return to reality, but Yuri has skipped through all the flags...?