Sisterly Vows of Revenge

Two sisters marry into their family's greatest enemies to seek revenge for their parents' tragic deaths. As one falls for the Grant heir, the other plays a deadly game of deception, where oaths sworn in blood lead to betrayal. Whose heart will remain unbroken?

Prologue
My sister and I were married into the enemy's house on the same day. She became the primary wife, while I was made a concubine.
She constantly reminded me, 'We're here for payback. Don't you dare fall for the Grant Heir.'
I followed her advice, drinking birth control concoctions daily, keeping watch over my husband, and poisoning our in-laws' meals until they finally perished.
Yet at the funeral, she betrayed me in the name of justice:
'Our husband treated us so well. How could you bear to kill his parents?
'No matter how great our grievance, hasn't the love we've shared these days been enough to erase it?
'Why wouldn't you listen to me and forget about the past?'
In the end, she killed me with her own hands, offering my severed head before our in-laws' spirits.
'Your daughter-in-law has avenged you. May you rest in peace. It's a pity you won't see your grandchild born...'
Only then did I realize my sister had never taken the birth control.
She had fallen for that man and, fearing discovery, sacrificed me to earn his forgiveness.
When I opened my eyes again, I was back on our wedding day.
This time, I would exact my revenge on her as well.