Am I overreacting?

Ok, so on Friday we got our TY pics, so I spent Saturday writing out all the TY's - the deal was my DH would seal and stamp them so we can get them out this weekend....last night my MIL comes over and sees the stack of envelopes on the table - and says 'oh you got your thank yous?' I look up to see her opening and removing my parents' thank you card from its envelope! She claims she just wanted to see the picture and wasn't planning on reading anything - but still, I say she crossed the line going through our stuff, especially a card clearly addressed to my parents. DH just shrugs and says 'well, what are you going to do!'
I am really ticked - I realize I am typically overly sensitive when she helps herself to anything in our house (food, beverages) b/c that's not what I'm used to at all - but this really annoyed me.
What would you do?

yup... sounds like something my MIL would do. she is SO NOSY!!!!! the complete opposite of my DH, who is SO private! do you believe that she had the nerve to ask him what my parent's gave us for a wedding gift?! MYOB LADY!!!!!!!! she just wanted to know who gave more.

so one day we were on the phone and she kept making little comments and totally digging for info. so I said 'my family and my side were SOOOOOOOOO generous' and they WERE! MUCH more so than her side. she says 'well my family was generous too' and I didn't say anything back. that shut her up for good.