The answer is - must be - quite simple.
Because she knew who had poisoned him and she was willing to do anything - endure anything - rather than let that person be suspected.
"There is not far to go now. Who could that person be?
Would she have shielded Philip Blake? Or Meredith?
Or Elsa Greer? Or Cecilia Williams?
No, there is only one person whom she would be willing to protect at all costs." He paused. "Miss Warren, if you have brought your sister's last letter with you, I should like to read it aloud."
"Angela Warren said,
'No.'
"But, Miss Warren -"
Angela got up.
Her voice rang out, cold as steel. "I realize very well what you are suggesting.
You are saying - are you not? - that I killed Amyas Crale and that my sister knew it.
I deny that allegation utterly."
Poirot said,
"The letter -"
"That letter was meant for my eyes alone."
Poirot looked to where the two youngest people in the room stood together.
Carla Lemarchant said,
"Please, Aunt Angela, won't you do as M. Poirot asks?"
Angela Warren said bitterly,
"Really, Carla!
Have you no sense of decency?
She was your mother - you -"
Carla's voice rang out clear and fierce.
"Yes, she was my mother.
That's why I've a right to ask you.
I'm speaking for her. I want that letter read."
Slowly Angela Warren took out the letter from her bag and handed it to Poirot. She said bitterly,
"I wish I had never shown it to you."
Turning away from them she stood looking out of the window.
As Hercule Poirot read aloud Caroline Crale's last letter, the shadows were deepening in the corners of the room.
Carla had a sudden feeling of someone in the room, gathering shape, listening, breathing, waiting.
She thought: "She's here - my mother's here. Caroline - Caroline Crale is here in this room!"
Hercule Poirot's voice ceased.
He said:
"You will all agree, I think, that that is a very remarkable letter. A beautiful letter, too, but certainly remarkable. For there is one striking omission in it - it contains no protestation of innocence."
Angela Warren said without turning her head,
"That was unnecessary."
"Yes, Miss Warren, it was unnecessary.
Caroline Crale had no need to tell her sister that she was innocent, because she thought her sister knew that fact already - knew it for the best of all reasons.
All Caroline Crale was concerned about was to comfort and reassure and to avert the possibility of a confession from Angela.
She reiterates again and again - 'It's all right, darling; it's all, all right.'"
Angela Warren said,
"Can't you understand? She wanted me to be happy, that is all."
"Yes, she wanted you to be happy, that is abundantly clear. It is her one preoccupation.
She has a child, but it is not that child of whom she is thinking - that is to come later.
No, it is her sister who occupies her mind to the exclusion of everything else.
Her sister must be reassured, must be encouraged to live her life, to be happy and successful.
And so that the burden of acceptance may not be too great, Caroline includes that one very significant phrase: 'One must pay one's debts.'
"That one phrase explains everything.