I'm thinking of changing the password to my sister's computer
Why are you going to change her password?
She is a bitch
It is wrong to change other peoples passwords to lock them out of their devices. If you have a problems with some on you should tell them and not tamper with electronics.
It's not like she will use the computer when I change the password.
She will probably need to use the computer for work or something. There is no reason that you should change the password when the computer does not belong to you.