If she brings it up, I would stick to the fact that she went snooping thru your belongings. First of all, that's just wrong. Period. Secondly, it's just like eavesdropping -- guess what, you might find out something you would rather not have known, so if you do, that's your problem.
And I would probably nix the overnight visits bec I am not going to have someone snooping my house. or....better yet, I'd plant some very interesting items for her to find while snooping. Very interesting.
Then again, I'm the daughter of a couple who got mad at a SIL for snooping in the medicine cabinet. They confronted her, she denied it, even her dh called her on it bec she talked to him about what she'd found. So my father slightly pulled the cabinet out of the wall and drilled a hole in it. He then taped it shut and filled the cabinet with marbles. Put it back into the wall, took the tape off, told all of us not to touch it. SIL comes over that night, goes to the bathroom and 2 seconds later....the sound of marbles falling all over. That put an end to her snooping.