Geeze loueeeze this makes me so glad I don't work in Retial anymore and it makes me so grateful for my guvmint job (tho office politics there can suck too).
I would do a few things.
1st check out your State's harassment and stalking laws. Huh? Yeah.. in there there may be something (like going thru your stuff or trying to get you in trouble at work) that classifies as harassment. It may be a long shot but heck.. you can do it by reading the law.
Second, get yourself very well versed on what your can and cannot do as her supervisor. For instance, if she is pilfering thru your office and your desk, let her know you expect this to stop. IOW let her know you know and your space is not hers to go thru. then lock it. If that means buying your own file cabinet with your own key and putting things in it and locking, then do so. It may be a pain, but you are protecting you.
Write down everything she does that is not in your best interest or is borderline insubordinate, complete with dates and times. If you have to go to the legal system complete information is important.
last, if you can, suggest to managment that a camera be installed that watches the cash area to see exactly WHO goes in where that is. If more $$ is missing, the camera doesn't lie. This action will have to take place when these two are not there and un beknown to them and other employees for it to work.
Last, if she has a computer on her desk and there is inter-office email, review the computer policy where you work and then MONITOR her computer. yes. That is legal at a place of employment and anyone inappropriately using a company machine is up for charges (and if may help with the harassment thing too).
Best luck and don't let this turkey get you down.