First and foremost, I don't understand why you can't simply deny access to YOUR computer by your sister and HER friends? But I'm not in your situation and therefore can't speak on that subject...

Bottom line is, if you are going to use P2P software, you are putting your equipment at risk - not only from the files/virues/spyware/general **** that will be downloaded, but also you could bring the heat from the RIAA and others in the area of illegal downloading of music, etc. This is a BIG risk you are taking...

All that being said, you shouldn't let this happen - but if you must - lock down the accounts for your PC tightly, like you mentioned... Give her an account that only has write access to an absolute minimum on the machine. Use your account for yourself, log out when you are done... Make her use her VERY limited account... Use secpol to limit what can be run, etc...

Use of P2P software is just asking for trouble... I'd fight tooth and nail to ensure that someone else couldn't use MY machine to perform actions that are just asking for trouble...

I want you to understand the risk you are taking - IMO, it's not worth it at all....