Children's literature is a fascinating world of colorful imaginations, vivid dreams, and endless possibilities. Often, it covers a vast range of concepts that include familial settings, friendships, adventures, fantasies, and education. One such frequent theme is the arrival of a 'new brother,' a trope that has been alluring both authors and readers alike.

The purpose of introducing a new brother character often serves to instigate change in the story. The concept of a new sibling, in general, tends to disrupt the existing family structure and relationships, creating space for an array of emotions, including excitement, curiosity, jealousy, fear, and even anger.

From a psychological standpoint, this situation allows children to navigate through intense feelings, thereby fostering their emotional growth. As children read these narratives, they come face to face with characters reeling under the influence of a sudden change in their life. They learn to empathize with the protagonists, hence developing the ability to understand and share feelings of another.

Literature masterpieces like E. Nesbit's 'The Railway Children' or Louisa May Alcott's 'Little Men' beautifully portray the introduction of a new brother in the family dynamic. The characters demonstrate a broad spectrum of emotions, including bewilderment, misgivings, acceptance, and eventual bonding. These stories have helped readers understand and accept the idea of changes that a new sibling brings into their lives.

New brothers in literature, whether human or of the animal persuasion in books like 'Charlotte's Web', help children come to terms with changes in their relationships with parents, friends, and personal space. It encourages them to learn about sharing, power dynamics, and coming to terms with their feelings.

Reading about these experiences assists children in understanding their feelings and expressing them constructively. They learn to make sense of their initial reluctance towards a new sibling and gradually accept the change, paving the way for lifelong loving relationships.

In conclusion, the new brother motif in children's literature is more than just a plot tool. It serves as a psychological bridge connecting children to the understanding of relationships, developments, and changes in one's life. It ensures a healthy emotional development, equipping children to face and accept the constant changes that life presents, much like the arrival of a new sibling.