Steam – Is it possible to make steam detect that I’ve copy-pasted the dlc into the game folder

To copy Steam game files from another computer to save you having to download the full game, simply do the following;

• Cancel the download on your machine and delete local files for the game
• Close Steam on your computer
• Copy the whole folder Borderlands 2 from your brother's PC into Steam\SteamApps\common
• Delete the .exe since every user has it's own .exe if Steam DRM is used (what BL2 in fact does)
• Tell Steam to download Borderlands 2 and it will discover the existing files
• Verify files to get your own .exe and make sure nothing is corrupted

You could try this:

1. Move the game folder to a different location.
2. Click on Delete Local Content for the game in Steam if available.
3. Initiate the installer.
4. Once it begins downloading, pause and close Steam.
5. Move the original game folder back into where Steam is downloading the new files.
6. Run Steam and if it doesn't automatically do so, verify file integrity.