Responses
Well because that was the original plot of the story that the MC's reincarnated. It's a natural course of event. She only managed to extend his life but probably can't change the flow of the plot. Afterall she said that they are already near the start of the original story since the MC heroine was about to enter the palace.

I read the spoiler that crown Prince will die and then his mother will also die, only his younger sister will survive.
Wtf author why you kill crown princeeeee