I suddenly became a count's daughter and gained an older sister! Wait, aren't both my stepfather and my older sister's fiancé total scumbags??

Due to her mother's remarriage, Aria became a countess's daughter. While she gained a wonderful older sister, her family—including her biological mother—are all terrible people, and even her wonderful older sister's fiancé is a dreadful person. She must somehow save her sister! Aria, who learned reading, writing, and arithmetic at Sunday school, helps her older sister by assisting with work!