The question of Morozko's brother's name in Katherine Arden's Winter of the Witch is a bit tricky. The novel doesn't explicitly name a brother for Morozko. While the book delves into the complex relationships and family dynamics within the ded (the spirit world), particularly focusing on Morozko's position and power, it doesn't introduce a sibling character for him.
It's important to distinguish between the narrative's focus and the potential for broader Slavic mythology. Arden's Winternight Trilogy draws inspiration from Slavic folklore, but it's a fictionalized interpretation. While some characters and elements have roots in established myths and legends, the author has creative license to shape the story. Therefore, the absence of a named brother for Morozko within the novel doesn't necessarily mean one doesn't exist in the broader context of Slavic mythology, but it is absent from Arden's specific telling of the story.
The novel focuses on Morozko's role as a powerful ded and his relationship with Vasilisa, highlighting the intricacies of the spirit world and the complexities of power dynamics within it. The lack of a named brother is likely a deliberate narrative choice, serving the overarching plot and character development rather than detracting from it. Introducing a brother might have shifted the focus away from the established central relationships.
To summarize, based solely on the events and characters presented in Katherine Arden's Winter of the Witch, Morozko does not have a named brother. Any speculation about a brother would be conjecture based on external sources, not the events within the book itself.