This is a recent development with kindles. Some publishes allow you to share books. My understanding is that it is just once and only for 2 weeks or something.
You can loan/borrow but it's only certain books and it's limited to 2 weeks. (I've never done it so that is just my understanding).
HOWEVER, I ordered my MIL a kindle under my Amazon account. Before transferring it to her, I could have downloaded all of the books I have on my kindle to hers, then transferred registration to her. So your SIL could order it under her account. Or even transfer registration to herself, download the books and then transfer registration back (I think -- never done it more than once so you would want to confirm that).
Re: One more kindle?
You can loan/borrow but it's only certain books and it's limited to 2 weeks. (I've never done it so that is just my understanding).
HOWEVER, I ordered my MIL a kindle under my Amazon account. Before transferring it to her, I could have downloaded all of the books I have on my kindle to hers, then transferred registration to her. So your SIL could order it under her account. Or even transfer registration to herself, download the books and then transfer registration back (I think -- never done it more than once so you would want to confirm that).