I also am trying to figure this out. I have an office iMac and an Air, and want to have my iPhone synced with all my music.
For your setup you may want to try
syncOtunes. Or for a more thorough sync try
ChronoSync. Though I can not attest to either products reliability. As any synchronizing I do is manual.
Since I never had to worry about a laptop I used to have a main computer with the iTunes library. For myself and my roomates we only used that computer for purchasing music or adding CD's. On all the other computers I changed the iTunes music folder to the main computers. Simply by mounting the network volume then selecting the iTunes Music folder on the server.
Then on each computer I would simply move the iTunes Library database file and replace its respective database file. I would do this every 1 to 2 months when music was added. Mainly since we did not add much music after inputting all of our massive CD binders, and saving then readding each persons playlists was also a pain.
On each machine automatic update was disabled to make sure each was running the same version. Only the server was allowed to organize the library. I did this instead of iTunes sharing since I wanted each computer to be able to burn CD's and make playlist's
As to the OP's idea for FTP from your NAS that could work for allowing your sister to grab the new songs. However, you will need a static ip address from your ISP and tweak your routers firewall to open the appropriate ports to your NAS. Your sister should then be able to access the files by typing the NASes address into an FTP program like Cyberduck. Then add the songs to her library once copied to her Mac. Though all of this would be a lot of fuss.
Personally I would just copy over the 'Music' folder. Which should have your current database file, artwork and all of your music. Then when she comes to visit just use a program like syncOtunes to synchronize your libraries. Be sure to save your playlists incase the synchronizing software does not synchronize the database files.