on 04-29-2013 02:07 PM
You can become NookFriends and share books via the LendMe feature. However, many books aren't available through that and there is a limit of one share/lifetime of the eBook with it.
If you and your sister see each other fairly regularly a better option might be to simply side load each others books onto your Nooks (you'll each have to enter your name and credit card information on each other's Nooks). Another option would be to use the same account, then you'd each have the full list of books (but no individuality).