Can I share my Diablo 3 account?
I already have an account for Diablo 3.
Can my brother use my account, but play with a different charecter, and then we can both go online together?
Your account cannot be logged on from multiple locations. From a technical standpoint, this is because your account name is the unique identifier for your in-game session. From a pragmatic standpoint, one game license does not allow multiple people to play at once. If you attempt to log on while you are already logged on, the first session will be stopped in order to begin the next one.
Also, please be aware that it is against Blizzard's account sharing policies to allow another person to use your account information. Any issues that arise (eg. your brother harming one of your characters by accident) from sharing your account will not be covered.