You will need admin rights on the computer. So that you log on as admin, and she has to log onto the same comp as a user. This can be done by giving her her own profile.
Start menu, then to contorl panel, into control panel, select "user accounts" (bottom of control panel) and then make sure your the admin of course. Now add a user - name it with your sisters name, and give it limited access (means that they cannot do nearly anything without your say so).
now make sure YOUR account is password protected so that she cannot use the admin account, and if there is a guest account, make sure that its limited use as well.
Remember to make SURE yours is the admins account, and that you have it password protected.
From now on your sis won't be able to put anything on the comp. In fact- she can do next to nothing