Diego is the son of Santiago, who is Jose's brother. This makes Diego Jose's nephew. In relation to Carlos, because Santiago is Carlos's uncle (the brother of Carlos's father, Jose), Diego is Carlos's first cousin.

Maria is Carlos's mother and Jose's wife. Since Jose is Santiago's brother, and Luisa is married to Santiago, Luisa is Jose's sister-in-law. Diego, being Luisa's son, is therefore Maria's nephew by marriage. Since the relation is through marriage and not blood, Diego is Maria's nephew through her marriage to Jose.

Diego cannot be Fernando's son because Diego is the son of Santiago and Luisa. Fernando is single and has no children, therefore he is not Diego's father. Diego is Fernando's nephew because Diego is the son of Fernando's brother, Santiago.
Jose is married to Maria. They have one son named Carlos. Jose has two brothers, Santiago and Fernando. Santiago is married to Luisa, and they have a son named Diego. Fernando is single and has no children. Describe how Diego and Carlos might be related to Jose and Maria, respectively. Also, is Diego Fernando's son?

Provide a very brief response.