Final answer:
Casiopea's cousin in 'Gods of Jade and Shadow' is Martin Leyva, who plays a significant role in the family dynamics that are central to the story.
Step-by-step explanation:
In Gods of Jade and Shadow by Silvia Moreno-Garcia, Casiopea's cousin is Martin Leyva. Casiopea Tun finds herself on an unexpected journey with the Mayan god of death, Hun-Kame, who is not her cousin but rather a central figure in her adventure. The dynamics within Casiopea's family, particularly with her cousin Martin, play a significant role in the development of the story and add depth to the characters and their interpersonal relationships.