want to do anything
with "/etc/passwd vs.
Bad dog! No biscuit!
(Or if you prefer: "Abandon all hope, ye who enter here."
On a standard Linux setup, "each computer is the master of its own world," so if your sister changes her password on her
machine, then on her
machine "it is changed." But, nowhere else.
Yes, you can
do what many offices actually do, and set up an LDAP server and configure Linux to consult that server (instead of the shadow-files) to authenticate logins. The LDAP-server would have to be one computer that you know is always
on-line, and beware, because it would be difficult for any computer to log-in anywhere if that server isn't there. It would also be impossible to grab that laptop and take it "away."
So... maybe the best thing to do is to make peace with your sister, or let her do what she will. Unless
you want to set-up and learn about the configurations that large business networks sometimes do use. It might be a useful exercise, but it's going to affect everyone else in your family.