Ane ni Naritai Ane VS Yuri ni Naritai Imouto

One day, Shio arrives home to find that her mother has gotten remarried and is going on a business trip with her new husband, leaving Shio with her new stepsister Kafka. Being only a day older than Kafka, Shio is concerned that Kafka will try to take the big sister position from her. Kafka, on the other hand, is determined to make Shio, who was her first love, fall for her.

(Source: MangaDex)