Rikon Yotei no Keiyakukon nanoni, Reikoku Koushaku-sama ni Shuuchaku sareteimasu

Mila, a girl raised in an orphanage, was born with a rare ability—she can absorb magical energy from objects and people. Her adoptive father, Baron Weston, has exploited her gift to cleanse weapons and dark artifacts stained with miasma, wearing her body down to the point that doctors have told her she had only a year left to live. One day, Mila collapses from exhaustion at the temple and is rescued by a beautiful woman named Anneliese. But the help comes at a price—Anneliese begs Mila to save her brother, Darius, the Commander of the Holy Knights and one of the most powerful dukes in the kingdom, who suffers from a rare illness caused by an overload of magic in his body. Anneliese proposes that, in exchange for getting saved from the baron, Mila help Darius in a "certain way." (Source: Bunkasha, translated)