Preventing the installation of Chrome Addons

You can prevent Chrome from loading existing extensions, or installing new extensions by appending the --disable-extensions flag to the command that launches Chrome (e.g. via the properties of the shortcut icon):

chrome.exe --disable-extensions

To override the homepage, you could append the --homepage=<URL> flag:

chrome.exe --disable-extensions --homepage=https://encrypted.google.com

You could also try to set up an administrative policy to disable extensions (see documentation). It might be more flexible, but I can imagine that malware developers would modify these policies themselves.

Your sister has probably more to worry about if she blindly installs adware/malware. Some education about the risks of her behavior might be more effective than disabling extensions in Chrome.

You could use Spybot: Search & Destroy, which has a browser protection feature. It also protects the start page from being changed. You could also use Ninite to install and update some of the most popular freeware and open-source software adware-free.

I think the best way would be to change to firefox, since the opt-in feature is relatively foolproof (this window appears every time a external application installed an addon and if the user doesn't opt in the addon gets deleted): However I've heard that some malicious adware manipulates this dialogue, but this seems to be quite rare.

I would use Windows Access Control and Permission to prevent installation of Chrome extensions to Chrome's folder by removing the Write and Modify permissions of user accounts to this folder:

In Windows Vista/7/8, the folder is at this path:

%userprofile%\AppData\Local\Google\Chrome\User Data\Default\Extensions