It's because the ghost said he's (the prince) gonna be poisoned or die and she can't just ignore someone dying because he was someone (from childhood) that showed her kindness
This is what i remember, correct me if I'm wrong.
Okay, I'll try my best to explain what has happened until now. At the beginning, our MC was reincarnated as the first daughter of a Duke. The "ML" (I'm assuming he's the male lead) is the second prince of the empire.
They actually first met when they were younger. They encountered each other in the palace, and like the person above said, he showed her kindness. For that reason, she wanted to protect him.
As for why she pretended to be an old lady and the granddaughter of that old lady, it is because she was able to see, hear and interact with ghosts. In the empire, that was called sorcery and sorcery is banned. Of course, our MC didn't want to risk the lives of her family, especially her younger sister's, so she pretended to be a lowly commoner in the hopes of being able to save the prince.
She first heard of the plan of his murder through a ghosts. She's masking her ability to see ghost as sorcery or the ability to see "profecies" or "predictions". Right now, both of our MCs are working together to catch the the one(s) plotting the prince's murder.
I hope I was able to help.
Ok so i forgot but shes a princess right? And im assuming hes an emperor or something. How did they meet again? I know she sought him out pretending to be the old woman but i forgot why