Thank you all for your responses.
The limitation on weight certainly creates issues with the Kindle, since she cannot hold a large hardback book for extended periods. Admittedly, a bookchair looks as though it would help with the issue of the arthritis in her hands.
I looked at the bookchair, and it appears from the images that it would have to rest on a table or other rigid hard surface.
The problem we run into is that she cannot sit at a table indefinitely, which limits the utility of the bookchair. The arthritis in her knees and back will not permit her to sit in a chair rigid enough for her to sit at a table for long enough periods. At this point, the only places she can relax for extended periods is in her reclining easy chair (motorized to help her move from sitting to standing positions and back again) or in her bed (with the Tempurpedic mattress to absorb joint stress). So she must be able to hold onto the book herself (unless, I suppose, there is a bookchair or cradle equivalent that has a lapdesk-type base for her to use in the recliner).
All of which brings us back to: is Cybook the best option, do you think? Just downloading straight from fictionwise and similar sites directly onto the Cybook, without conversion?
I wish I could manage the downloads for her, but I live in another state.
She REALLY loves books, and I would hate for her growing health limitations to deny her those fundamental pleasures in life.