I remember the main character was at one point talking to her sister who was going to marry the male lead but the sister convinced her that the male lead hates her and wants to kill her.
So the sister gave the main character poison to drink. She drinks it, no surprise there and dies, but then travels back in time not knowing that the male lead actually loves her.
This is almost certainly Revolutionary Princess Eve, also identified here. It is licensed by Tapas.
Princess Evienrose, despite advocating fiercely for the rights of the homunculi rebels, now awaits death at their hands… that is until she is poisoned by her own sister! Awakening eight years in the past, Eve now has one goal: become the Empress and liberate the homunculi to stop the rebellion from ever happening! To do this, she appoints the rebel king himself, Mikaelis Agnito, as her personal knight. But with Eve’s scheming sisters standing in the way, the fight for the crown is on, and the fate of the Hadelamid empire rests in her hands!
The poisoning happens in chapter 2; the main character's sister tells her that, as part of the aftermath of the revolution in the synopsis, she will be brutally killed by the leader, because he hates her. She gives her some poison so it will at least be a painless death. It is then shown that the sister just wanted to get rid of the woman the leader loved and take him for herself.
