You are actually lucky to be using Win2k in this situation - but you have to take advantage of the strengths of the OS.
Set up a new user for your sister, and/or other family members, to use when "visiting" your PC. Set a password, and use it, for your main account as well as for the new account.
Set this new user's rights to either "User" or "Power User" so they can use the apps they need.
The key here is to get into the habit of logging off of your main account when either not using the PC or before turning it over to another user.
Once the new users log in this will give the new account their "own" directories (My Documents, etc...) and forbid access to "your" directories.
I hope this helps.