I have already filed my taxes and received my return -- how can I file for my brother who died in November. Can I file for him on my account and, if so, how?

When you use online TurboTax software you get one return per fee.

Each return needs its own account and user ID.   If you use the same account and user ID for a second return, the second one overwrites the first return and it is lost forever.  

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

If you are going to use the TurboTax online editions, you will need to create a new account with a new User ID for his tax return.  The same email address can be used on up to 5 online accounts.

Go to this TurboTax website and click on Sign up to create a new account - https://turbotax.intuit.com/