Delete the rewire.bundle files from here:

/Library/Application Support/Propellerhead Software
/Users/YourUserName/Library/Application Support/Propellerhead Software

Also delete the PlogueEngine folder from here:

/Users/YourUserName/Library/Application Support/Avid/Sibelius 7

Delete the contents of the following folder, if they are there (these are the logs Sibelius creates when it initialises ReWire. If these aren't present, don't worry):

/Users/YourUserName/Library/Application Support/Avid/Common

Empty the trash and start Sibelius 7 again. To check Sibelius is able to be hosted as a ReWire slave:

1. Click Play > Setup > Audio Engine Options
2. Click both Repair and Clean-up, if you can, and quit Sibelius 7.
3. Start Sibelius and check the Repair and Clean-up buttons are now disabled

Now quit Sibelius, start the ReWire host again and check to see if you can start Sibelius 7 using ReWire.

If you find Sibelius 7 crashes during start-up, check it's not running in 32-bit mode:

1. Go to the /Applications folder
2. Select the Sibelius 7 app
3. Go to File, Get Info
4. Untick Open in 32-bit mode
5. Close this window and try running Sibelius again