You can't have two accounts on your nook that I"m aware of. You can have more than one profile on your nook, but it will be under one B&N account. You can register more than one nook on your account but you can't have more than one account on your nook. If you were to register your nook under a different account, you would not have access to the books from the original account. However, it's possible your sister could "lend" you the books to read if you decide to go forward with setting up your own account. That would save you from having to repurchase the books. Keep in mind not all books have the LendMe technology and you only have 14 days to read the book.
From B&N frequently asked questions:
With our LendMe technology, you can lend and borrow NOOK Books purchased on BN.com and read them on your iPad, iPhone/iPod Touch, Android, Blackberry, PC, Mac, or other compatible device. Any book that can be lent will have LendMe icon next to it.