Alright, I seem to finally have figured this one out. I've been creating, removing, joining and leaving Homegroups all the weekend and by the trial and error I've summarised my findings as;
When you share something within a Homegroup (be it the standard Music, Documents... in the Homegroup management tools or any library), the shares are also involving the Everyone group. Looking in Advanced Sharing I can see the Everyone group getting a permission setting of Full Control when sharing a library using right-click -> Share with -> HomeGroup (Read)/Homegroup (Read/Write). Setting the sharing to None, brings up a padlock on the folder within the library and noone has yet again access to the folder/library.

In effect, this means that my gf can access the shares in pretty much any manner she pleases, given that she can access my server box by ie. UNC-path. It's weird how this is getting set up by Windows when managing Libraries. Sure, it's easy as pie to create and remove sharing of a Library, but if Everyone gets the access Full Control, even when I set the Library to be shared as Homegroup (Read), why do I have the option to only set it to Read?