I just remembered this manhwa. I think its lost with my reading list last time.
The mc is a duke's daughter who went missing. But actually (i think) she was kidnapped by another dude and was raised to kill the duke. The other dude told her to pretend to be the duke's daughter and kill him but she didn't know she is actually the real daughter and killed the duke. She was killed, found out about everything and went back in time.
When she went back in time she tried to change everything but could not return to her family due to a curse. She also met the crown prince which was how the duke's family found out she was alive. She did not have a sibling before she regressed but now she does.
Oscar or the emperor from my daddy hides his power. They both dont deserve to die quickly and easily. Both deserve to be sliced through their flesh and and skin day by day as if you're cutting an onion (╬ ̄皿 ̄)凸