How to make items copied into a folder respect its permissions?
There's an option for and access control entry (the thing you created in Get Info) to be inherited, but Get Info doesn't give you the option to enable it.  Also, you'll want to make sure you have full access to items she's added/edited there, and the easiest way to do this is to create a group for the two of you, and allow access to all group members.


Create a group (in System Preferences -> Accounts pane), and check the boxes to make both yourself and your wife members.
Open the Terminal utility, and run the command:

chmod +a "group:admin allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" /path/to/the/folder


(replacing /path/to/the/folder with the unix-style path to the folder you want to grant access to.  If you aren't sure what that is, just paste in the rest of the line, including the space before /path/to/the/folder, and then drag-and-drop the folder from the Finder into the Terminal window, and it'll paste in the path for you.)
The access control entry you just created will be applied to new files and subfolders created inside that folder, but won't apply to the files and subfolders that're already there.  So, Get Info on the folder and use the Apply to enclosed items option to apply it to the existing items.