I messed up and redeemed the multiplayer online pass while logged in on my sister's profile and now I can't play online with my saved character. I tried transferring my save files to my sister's account, and it seems there are two kinds of files on the profile: the actual save files (which are copy-able), and a locked kind of file. I copied the save files, but the game doesn't recognize them. There's only the "new game" option in the menu. It seems those save files are not enough—it seems I also need the locked file to be copied. How can I transfer these save files?
The Online Pass isn't locked to a single save/user, once it's installed on a console it will be available for all users who play on it.
Go to your sister's account and log on to the Playstation Store, if you redeemed the code while she was logged in, the online pass should be available on her download list, download and install (shouldn't be more than 1MB), then go to your account, you should be able to play online.