My mom the spy...

So I'm kind of a calm angry right now.  My sis just called me awhile ago and said:  "I don't want to get in the middle of this (the whole thing between my parents and me dealing with Derrick), but you might want to change your password.  But please, please, please don't say anything to mom."

So I guess when I was home over the weekend, my email password got saved to the computer and she's been snooping through all the emails between Derrick and I.  I was a little bit angry.  I'm not even really angry at the fact that she's snooping through my personal emails.  It's more that while I was home she acted like she was perfectly fine with the whole situation.  She was smiling and acted like it didn't bother her anymore; but obviously it does.  I just wish she would confront me about it instead of going through my shit.  I've made it very clear that I'm willing to answer any questions she has or explain things to her more, but no.  She would rather resort to spying. 

I have no clue how to resolve this situation...