Can you restart the computer into safe mode
Tap the F8 key as the system is restarting, right after the single beep
Choose safe mode from the options on the screen
When you get a choice of which account to log into, select the Administrator account
If it asks for a password, by chance one may not be set, hit Enter on the keyboard
If that gets you into the Admin account
Go into User Accounts in the Control panel
Select your sisters account and delete the password
Does that help?