The fact that the Kindle Fire has more potential as a tablet than an eReader is interesting. I have a Kindle eReader and it is much easier reading on those E-ink screens than an LCD. If you plan on only
reading on the tablet (lots of people seem to do this), don't get a tablet. Get an eReader like the Kindle or the Sony one (which is a bit pricier but nice). Not to mention the Sony eReader is actually Android in E-ink!
Indeed, if all you do is read books then get a E-ink powered device like the original Kindle or one of the E-ink Nooks like the Nook Touch.
As for Sony having a Android driven OS in the background, this isn't too special... the Nook Touch / original Nook have Android aswell, which is kinda funny trying to play Angry Birds on a E-ink device once hacked...
The 1fps E-ink screen has biiiiggg trouble dealing with motion-content like animations. xD
Basically... summing things up...
1. If your sis wants to read only... get a E-ink based device, it will be super power saving as the E-ink device only refreshes the screen ONCE when requested to, means, there ain't a "FPS" value with those... it's a on-demand refreshing, which is extremely powersaving, however awful if you watch movies, etc... as you will get a 1-2fps experience.
2. You want the full fledged tablet experience? If you are a cheapskate like I am... get a Nook Color if you want things immediately (because of the already existing awesome community) or a Kindle Fire (for the soon to come as awesome community, due to the open bootloader it has).
Should you have some more cash to invest, you might aim for one of the ICS tablets that are coming soon.