Unfortunately, your in-game profile is tied to your Steam account - there is no way to create a separate profile without purchasing another copy of the game. Most multiplayer games (especially on Steam) are this way - a unique identifier is assigned to your account for multiplayer purposes (often called a STEAMID or Guid, depending on the game).
This is done for a number of reasons. For instance, this allows Valve and game server operators to uniquely identify their clients, therefore this system is the basis for many ban and admin rights systems. Being able to create new multiplayer profiles or identities would create loopholes for people to abuse this system.
As Steam accounts are intended to be used by a single person, (the subscriber agreement states "You may not reveal, share or otherwise allow others to use your password or Account") there's no system in place for allowing for multiple multiplayer profiles under a single account. This is against the terms of the subscriber agreement, so there's no support for it.
If you and your brother both want to play the same copy of Modern Warfare 3, you're going to have to live with the fact that you're going to share a profile.
You should also be aware that part of the agreement also states:
You agree that you will be personally responsible for the use of your Account and for all of the communication and activity on Steam that results from use of your Account.
Therefore, if the other person playing on your account misbehaves/cheats, and this results in your account being suspended or banned from online play, you cannot say "I didn't do this, it was someone else on my account, please unban me." You are responsible for their actions as well as your own.