You have to maintain BRAND in material master and from there it will come in Sales order.
Check what field in article master you are using for "Brand", you may find those fields on tab "Sales B" of item in sales order,
If you didn't use any of above fields for "Brand", then you may need an enhancement.
Try user-exit MV45AFZZ (USEREXIT_MOVE_FIELD_TO_VBAP).