When I investigated Roon it seemed to me that Roon would simultaneously group itunes and the NAS without having to switch between them. This would allow full access to her music libraries. Can anyone verify this level of functionality before I suggest she explore Roon to her? Those with a Pi unit how has Roon worked for you? Thanks in advance.
Yes, Roon will do what you're looking for if properly configured.
Roon will let you set up multiple local sources, as well as Tidal, Qobuz, or Dropbox. Then when you search it will search from all of those from a single search box. When you navigate around in Roon, you can either be navigating all those local and network services at the same time, or only your local sources, or only a network source. For example, the "Overview" main page will show my local albums and artists, as well as new stuff that is being featured by Qobuz, plus some recommendations by Roon. But if I click on the "Qobuz" menu item it will only show me Qobuz stuff, and if I click on the "Library" menu item it will only show me my local sources.
Local Roon sources can be either a local or external hard disk on the Roon Core computer, or a network share mounted by the Roon Core server. So if your sister has all her iTunes music stored on your Mac hard disk, then you'd want to install Roon Core on the Mac and tell Roon to include the local iTunes music folder and also to include the NAS network share music folder.
That being said, the Roon Client UI can eat up a lot of CPU. You'd either want to make sure your sister's Mac is powerful enough, or think about running Roon Core on a dedicated computer that mounts the NAS and Mac music folders over the network. We run Roon Core on one computer, and use the Roon Client on our Macs, iPads, and iPhones to send music to different playback systems.