There are loads of free email services but the majority only allow you to access your mail via the web (rather than by having it arrive in Outlook Express, or whatever other email client you use).
The most popular provider which does allow you to access your mail with Outlook Express (etc), as well as via a web page, is Google's Gmail:
http://mail.google.com/
My own preference, though, is to use Gawab (which also allows both methods of accessing your mail). For instructions on how to set up a Gawab account (and how to configure Outlook Express to receive mail from it), see paragraph 3 et seq in my post here:
http://www.theanswerbank.co.uk/Technology/Inte rnet/Question407429.html
Once you've set up a Gawab (or Gmail) account for your sister (and you've configured your email client accordingly), all her mail will arrive in your Inbox. It you'd prefer to place it in a separate folder (and you're using Outlook Express), right-click on 'Local Folders', select 'New Folder' and give it an appropriate name. Then go to Tools > Message Rules > Mail, to create a rule directing her mail to that folder.
Chris