Hi - can I prepare another person's tax return under my account in Turbo Tax? I am preparing for my sister and trying to see if I can do it under my account or not

If you are filing online do not try to file someone Else's return in the same account. Online version can only hold one federal return. If you are filing for someone else you will have to make a new account for the person you are filing for or have them make the account.

You can use the same email address for notifications  but you each must have your own UserID and account.