Firstly, it could be an ad which has been inserted by your sister's email provider. If the advertisement content is sourced from a server which is currently inaccessible, it won't appear (but you probably won't want it to anyway!).
The same problem could occur if your sister has linked to an image (rather than embedding it) and the link is currently down.
If your sister has actually attempted to embed a picture within an email you might need to ask her to check the settings in her email client. If it's Outlook Express, she should go to Tools > Options > Send and click on 'HTML Settings', to check that 'Send Pictures Within Messages' is selected.
However you might have a default security option enabled in your own email client. (You've not stated what you're using. Outlook? Outlook Express? Windows Mail? Thunderbird? Something else?). This applies to Outlook Express:
http://support.microsoft.com/kb/843018
Chris