Diego is Sofia's brother. Sofia is married to Juan, and they have two children, Carlos and Mariana. Diego is married to Luisa, and they have a daughter named Isabela. How are Isabela and Carlos related to Juan and Luisa, respectively? Also, how is Mariana related to Diego?

Respond very briefly.
Isabela is the daughter of Diego and Luisa, which means she is not directly related to Juan by blood. However, since Juan is the husband of Diego's sister Sofia, Juan is Isabela's uncle by marriage. So Isabela is Juan's niece by marriage.

Carlos is the son of Sofia and Juan. Sofia being Diego's sister makes Diego the uncle to her children, which includes Carlos. So, Carlos is Diego's nephew by blood.

Mariana is also the child of Sofia and Juan, and like Carlos, she is Diego's niece by blood, because her mother Sofia is Diego's sister.