There is another consideration you may want to take into account. Even though your sister's iBook is still officially supported, it will phase through the 'vintage' stage into 'obsolescence' within the next 2 years. But as it is, Apple already isn't supporting Leopard with security updates any longer, and various other web-related software is no longer updated either. If you depend on any of this for secure web connections, you must realize that a lot of it has already been compromised. For this reason many web sites require Intel-based Macs to run their client apps.
I can say this from my own experience working mostly on a last model G5 iMac. It's only because of several Intel based MacBooks in the household that I can do those things the PPC Mac won't allow me any longer. Suffice it to say that I'm thinking more and more about upgrading my work horse Mac. Meanwhile, iTunes will run fine under Leopard accessing the iTunes Store, and so will access to QVC (although I haven't tried to register there). But that may change with the next iteration of Mac OS X or iTunes, and quite likely by the time these PPC Macs fade into official obsolescence (late 2013).
