Pretty well everyone will recommend a laptop. They are just as capable as a desktop, can be used anywhere, and take up far less space.
If your sister has a reasonable level of proficiency, and her existing PC has a USB port, she can easily transfer data from one machine to the other, using a USB memory stick that can be bought almost anywhere. Most supermarkets sell them, as do large newsagents, phone shops, etc.
If your sister isn't too sure how to do the transfer, we can talk her/you through the process in easy-to-follow steps here, in the forum. Just post in this thread and ask for help.
For your sister's needs, any of the machines mentioned here will fit the bill.