Hmm...
First a brief intro about
GUID (Globally Unique Identifier)...
Whenever u click "remember me" on login based site a cookie is created with a GUID in it...this cookie stores password and username in it, so that user can be identified on the basis of GUID on his/her return....
GUID is a unique string..
Now getting back to question....
When ur sis clicked on an AD or on
JS then those scripts executes and search for GUID in the cookie folder...
If they get one they stores it and return to there MASTER hehe in ur case to that prank....
Solution: clear all cookies... get the password change ur r safe then...
And too use Spybot search and destroy + AdAware to detect for any maleare present in ur system....