This is Don't Concern Yourself With That Book.
I was reborn as Ashley Rosé, the daughter of the 8th wife of the Emperor of Estria. ... Ashley Rosé was destined to die by the hands of the Crown Prince, Castor the Tyrant! “What does the empire mean to you?”, the Crown Prince asked, smiling oh so very beautifully. And then he killed me. After my death, I woke up once again in the past, only to be killed over and over again. As I continued to relive my deaths, I slowly lost my smile and changed. Let’s just try to survive first. “What can I do for my elder brother?”
The main character reincarnates as a blonde princess that lives alone in her palace. She has two brothers that appear in the story; a black haired prince that kills her for not answering questions to his satisfaction,
and a sickly man with light blue hair that is surrounded by plants. Presumably he has the ability to manipulate them in some way, but this isn't explicitly shown in the first two chapters.
The main character finds a book that tells the future, particularly of the black haired prince's plan to kill her.