Akuyume Reijou wa Ikka Metsubou no Yume wo Mita: Watashi no Mokuteki wa Ikinobiru Koto desu

Falitina, the daughter of a duke in a certain kingdom, lives a life of luxury until a "nightmare" changes her life forever. Misfortune befalls her, and then her precious Gemini dies. Upon learning of the tragic future, she vows to protect her beloved brother at all costs. I will protect you. This is the beginning of a tale of revenge that begins with a nightmare!

(Source: Pixiv Comic, translated)