Love You, with Death

"We were happy until that fateful day."
Born into a clan of apothecaries, Olive is a young girl who dreams of continuing her family's legacy. But then, one day, her hometown gets attacked. She loses her entire family, except for her older sister.
They get adopted by the lord of the domain, Earl Elvert, only to be treated like livestock. Olive does everything to protect her physically disabled sister, who's eventually taken hostage in order to coerce Olive to abuse her power.
"It's because only your family can make a 'perfect poison' causing instant death." That is, an ability to refine a special kind of poison from her blood.
Olive is forced to assume a false identity, marry and later poison the infamous "Murderous Duke." However, contrary to the rumors, Duke Victor seems to be an honest and kindhearted man…
"I don't want to kill him. But as things stand, my sister…" The more love she feels from Victor, the more her anguish grows.