See
How do non-US Kindlers purchase Kindle e-books?, which references
Amazon Kindle outside of the US. The trick is to get a US prepaid credit card with a legal US address, and use this as the 1-click purchase option on an account dedicated to the Kindle, but to always use gift certificates to actually buy Kindle ebooks.
If this is too complicated, and your sister does not want to buy MOBI ebooks from FictionWise and make them Kindle compatible with kindlefix.py or buy LIT (MS Reader) ebooks from anywhere and break their DRM, then the only other option is to sell the Kindle.