For ebooks it will be very similar experience with both. On Nexus you can read both main ebook formats, epub and mobi, with Kindle there's a restriction to mobi only. So if choice is between these two, I'd say Nexus. But, if your sister only wants to read ebooks, e-ink display devices are much better for the job. So make sure you won't buy her something that you want, but maybe she doesn't.
It's like you actually know me!
Thank you though. I think a Nexus is the way forward- like I said her laptop is also rubbish, and while I know it's not a replacement, anything is better than what she currently has and the Nexus does seem more versatile.
I am seeing her next weekend so I might test the waters a bit then, without tipping her off that I might get her one. She's had a really poopy time and she really deserves a lovely surprise and to be spoilt.