The brother in 'A Christmas Story' is an important part of the family fabric in the story. He's there during all the family interactions. He helps to create a more complete picture of a family at Christmas time. He might not have his own grand quest like Ralphie's for the BB gun, but his reactions to things like the leg lamp, or the general Christmas craziness, contribute to the overall comical and heartwarming feel of the story. His role is to be part of the family unit and show how siblings interact within that family during the Christmas season.
If it's Ralphie's brother in 'A Christmas Story', he plays a supporting role. He's part of the family life that Ralphie is in.
In the 'A Christmas Story', the brother is part of the family's Christmas experience. He adds to the family chaos and humor. For example, during family meals or when they are all together around the Christmas tree. He may not be the focus like Ralphie who wants the BB gun, but his presence enriches the family - based story and shows different aspects of family life during Christmas.