She can register up to six readers on the Sony account and share any books bought through the Sony account - but the Sony books tend to be more expensive. I use unswindle to strip the drm off kindle books that are cheaper than Sony books, which she can do and then convert them using calibre from .mobi to .epub and share with her sister.
I think it comes down to what kind of effort you're willing to put into the books. I don't mind stripping and converting files, but a less techie person might be overwelmed by it.
|