The only time the Console would ban you is because of you being on an account that has steam family shared from an account that is currently banned (your main). This is completly intentional and because of the nature of steam family sharing we can not allow an alt that comes from the banned main under any circumstances. If your Brother would like to continue playing on that account he would need to actually purchase Gmod on that account and not use steam family sharing.

The only time the Console would ban you is because of you being on an account that has steam family shared from an account that is currently banned (your main). This is completly intentional and because of the nature of steam family sharing we can not allow an alt that comes from the banned main under any circumstances. If your Brother would like to continue playing on that account he would need to actually purchase Gmod on that account and not use steam family sharing.