Nicole’s answer to “Is Sebastien’s love letter to Zoe typed? If it’s handwritten, how on earth does Mariana fail to rec…” > Likes and Comments

Maintaining the lie was easy since Sebastian had 2 personalities, as deducted from his early letters. Most probably one personality didn't know about the lies.

(view spoiler)[ I agree that I think the whole affair was all in Zoe’s head. In addition to what this comment mentions, I think Zoe just truly wanted to hurt Mariana by taking away the positive memory of Sebastian from her. The letter being typed is also an easy way for Zoe to forge these letters, whether she knew what she was doing or not. (less) (hide spoiler)]