Gojira - hacking into someone else's e-mail account is a felony, and I would absolutely report her and be insistent that it's investigated if she does it to you. I'm sure your grandmother wouldn't press charges, but you can. Many e-mail programs can tell you when the last time you signed in was (like for gmail, it's in the bottom right), so you can montior that if you think she might hack you. Also some programs let you ask for an alert if you are signed in from a computer other than the one you generally use (Facebook also has this option).
People have actually been convicted of felonies for this. If it happens, contact local police as well as the FBI, since it's a federal crime. Here is one example of an indictment of someone for accessing a co-worker's email account.
http://www.justice.gov/usao/pae/News/2008/jul/mendtepress.pdf
Don't be afraid of auntie screwing up your life. Screw up hers if she breaks the law!