Well there's a few recommendations.
Firstly - boot into safe mode - you can do this by rebooting your computer and as its loading up at the start keep hitting the F8 key, and then choosing safe mode.
When the log in screen comes up, hopefully you will see an administrator account that is usually hidden and often not password protected. Simply click the new administrator account, go to the control panel and then click on the user control part. Simply add a password onto the administrator account.#
I also recommend you use a hard to get password. I've seen a lot of people using simple easy to get passwords, date of birth is often a well used, and well known password, or your favorite tv show. Try using both numbers and letters, and both low case and capital letters. Try also using special characters and spaces.
My next recommendation is for if you leave your laptop logged in unattended a lot. My brother used to be always trying to get onto my computer so what I've always done is locked it. All you need to do is hit the start Windows key (the one with the windows logo on it next to the control ctrl key), and the L key, both at the same time. Pressed together, the start and L key will lock your computer, and mean you will need to enter your password again to get back in.
If you don't lock your computer, your sister could access your control panel, and remove or change your password.
If your wanting to catch your sister red handed, it sounds like you need a keylogger. Keyloggers are software that monitor and record computers. The problem with keyloggers is they often get caught by anti-virus software, as some computer users send them to other computer users, without their knowledge, so they can log their password.
According to Cnet's Download.com, LigfhtLogger is good. It has good reviews from Cnet themselves and good ratings from users (sadly no reviews yet though). It is free so it sounds quite handy and can be found at http://www.download.com/LightLogger/3000-2162_4-10705172.html?tag=mncol
Hope I can be some assitance to you.