Apart from getting rid of the 'bug', you said you wanted to get even, provided there is someone to get even with. I'd be careful. What you may want to do, is collect as much information about the system and what is installed on it, lets say by ghosting the image.
Quarantine the drive that the image is on. Now you can do a clean install, and have a record, should you be able to track an IP.

I'm not an authority on this, just an idea, albeit the fact that you'd need some resources.
Plus I would contact the authorities if you have a case. You just might help in nabbing someone that has done this to others. But you'll need proof, hence the need to document everything.

Sounds like your sis should have a word with her bank as well. They may be able to give both of you more advice, if at least to protect her account.

edit: sorry, you already contacted the bank....I'll pay better attention...