Grand Avail: Omamori no Madoushi wa Saioshi Last Boss Onii-sama wo Sukuitai

Mana, a sickly high school girl, faints while reading the final volume of her favorite novel, "Grand Avaire"—… When she wakes up, she finds herself reincarnated as Leticia, the younger sister of Serios, the ice prince who is the novel's final boss and meets a tragic end!! Still sickly even after reincarnation, Leticia resolves to avoid her own fate of death and save her beloved brother, Serios. Her days are filled with making overpowered charms, making contracts with fluffy spirits and talking holy swords, and much more—!? A fantasy about a magically-powered genius young lady who is doted on by her beloved older brother and tries to avoid death flags!!

(Source: Hobby Japan, translated)