what you need to do ...
1) open wallet.bitshares.org in a browser that has a javascript console ... i.e. firefox
2) open the javascript console
3) run:
aes_root = bts.ecc.Aes.fromSecret("PASSPHRASE")
aes_root.decryptHex('ENCRYPTED-BRAINKEY')
The ENCRYPTED-BRAINKEY is a hex string that you can extract from your backup file (which is in JSON format). It is located below "key": "encrypted_brainkey",
as value
This will give you the decrypted brainwallet in hex ..
use any tool to convert the hex into string ..
e.g. brainwalletX.github.io