"Question: Do I let her sister know about this or do I just let it go?"

I will assume from your post that you are not talking to the sister. If you are.. stop. If you are not.. don't bring her into it.

NEVER.. talk to the other sex about this. Even if it is someone close. Your Mom would be the only OK'ish thing. Be careful with that too.

Why does the sister email you?

Relax
Eat
Think
Act normal
React.. Smartly.
Do something different.
Emulate.
Do Work.

Lets get "RETARDED" in here.