One Kindle. Two accounts?

SWMBO has borrow her sisters Kindle for holiday reading. I already have a Kindle account (reading on my Nexus S). From a quick trawl on the web it looks like I have to de-register the Kindle from my sister-in-law's account and register to mine to download some books to it. I take it that all the existing downloads will disappear and my s-i-l will have to re-register the kindle and download them over again? Anyone been through this?

Willy.