You've not stated which email client your sister is using, but I'll guess that it's Outlook Express.
Outlook Express has a nasty habit of 'losing' the information it requires to connects to an email providers servers. (To be fair to Microsoft, it's not necessarily the fault of their software. Your sister's anti-virus software has to temporarily change the settings in Outlook Express, in order for it to be able to scan mail. Sometimes the two programs don't communicate properly, and the settings get scrambled).
First, check that you know the SMTP and POP3 server addresses for your sister's email provider. If she gets her email service from a major UK ISP, you can find them here:
http://www.kitz.co.uk/isp/dns.htm
(Otherwise, take a look at the support pages of the firm that provides her email).
Then open Outlook Express.Go to Tools > Accounts. Click on the Mail tab. If your sister only has one email account, it will already be highlighted. Otherwise click on the relevant account to highlight it. Click on 'Properties', then on the Server tab.
At this point I'm hoping that you'll find that the POP3 and SMTP addresses are incorrect. If so, simply correct them. It's also a good idea to delete and retype the (asterisked) password. Then click 'Apply', 'OK' and 'Close'.
Occasionally, the information you see looks to be OK but Outlook Express still isn't forwarding the correct details to enable it to connect to the SMTP and POP3 servers. In which case, you might find that what I've typed above won't work. If so, start as before but select 'Remove', instead of 'Properties'. Delete the email account, then go to Add > Mail, to recreate it with the correct settings.
Chris
(PS: If none of that works, the problem lies with your sister's firewall, which is blocking Ou