Your sister can make herself an Administrator and the kids Limited
Accounts, but if they have any computer smarts at all that is easy to
circumvent. Aside from that, here are a few things to try:

1. Put a BIOS password on the machine to prevent anyone being able to
boot Windows without your sister.

2. Set the boot order in the BIOS to boot the hard drive first. A Setup
(BIOS) password will prevent easy change of this option.

3. Set strong passwords on the built-in Administrator and your sister's
personal account.

4. Look at the Microsoft Shared Computer Toolkit for further user
restrictions:
http://www.microsoft.com/windowsxp/sharedaccess/default.mspx

I must tell you that all of the above can be gotten around by a person
with computer skills, time, and the requisite tools. Those things will
make it harder for a "regular" user, though. If your sister wants to do
any of the above suggestions, she should probably have a computer
professional come to the house and set things up for her.

You - and your sister, of course - should also be aware that the root
issue is not a technical one but rather an issue of respect, obedience,
and parenting. These are things that have no technical solution.

Malke