Uncertainty

The strangest thing happened today...
I went to talk to Ralph about the email, but he acted as though he hadn't seen me in ages even though I saw him only last week...
He acted like he didn't remember it at all... or the post it affair.
"Ralph, I'm scared..."
"Of what?"
"Well I got a strange email the other day... I was wondering if maybe you got one too"
"An email? who was it from?" He asked, which answered my question enough
"I think it was from the same person who left the Post-it notes"
"Post-it notes? Is someone harassing you? Is it that guy you wouldn't cheat on Dave with?"
I hesitated for a moment.  I hadn't thought about Todd in so long.  We became friends at work, and he propositioned me one day, and I turned him down.  He held it against me for so long, but I hadn't heard from him in a few years.  Once upon a time... I was a good wife. I realized my thoughts had drifted and returned to the conversation.
"No, I haven't heard from him in a while. You got a post-it too though! It was about Erin, you know our shared secret-" he cut me off
"Wait... what do I have to do with Erin?"
"It was our fault..."
"Nobody blames you..."
"That's because nobody knows..." I said starting to doubt myself
"I mean I know you hadn't told anyone yet at the time, but you told everyone afterward, and it really was just an unfortunate accident."
"But she was our sister..."I trailed off
"...We never had a sister" he said, looking at me with concern in his eyes
I had nothing left to say.  I left unsure of everything I thought I knew. I cried in the car on the way home. What just happened?
Has my brother gone mad to convince himself that he has no guilt?  Who do I turn to for answers when there doesn't seem to be a right one? What am I supposed to do now?
Am I the crazy one? Did I ever really have a sister?
Of course I did.  I know I did.  I don't care if he doesn't want to remember her; I won't let her memory die! Maybe if he's going to deny the truth I should just let it out like the mysterious email told me to...