One of our rules is "your account your responsibility." I'll even quote Steam's terms: "When you complete Steam’s registration process, you create a Steam account ("Account"). Your Account may also include billing information you provide to Valve for the purchase of Subscriptions. You are solely responsible for all activity on your Account and for the security of your computer system. You may not reveal, share or otherwise allow others to use your password or Account. You agree that you are personally responsible for the use of your password and Account and for all of the communication and activity on Steam that results from use of your login name and password. You may not sell or charge others for the right to use your Account, or otherwise transfer your Account, nor may you sell, charge others for the right to use, or transfer any Subscriptions other than if and as expressly permitted by this Agreement (including any Subscription Terms or Rules of Use)."

Now moving on I'm slightly confused why you let your older brother use your account when this game is Free to Play now. You let him use your account and he attempted to scam someone.