It could center around the idea of breaking free from the traditional role of a younger brother. In many families, younger brothers are often in a more subordinate position, so perhaps this novel is about the character's journey to change that and gain more autonomy. He might face various challenges, like family expectations and social norms, which he has to overcome to achieve his goal of not being 'a younger brother' in a metaphorical sense.
Well, it could also be about self - discovery. Being a younger brother might have hidden the true self of the main character. Through the story of the novel, he may be exploring who he really is without the label of 'younger brother'. He might be finding his own voice, his own dreams, and his own way of living that is different from what was expected of him as a younger brother.