Juan is the son of Maria, who is Carlos's sister. Therefore, Juan is Carlos's nephew. Eduardo is Carlos's brother and has a son named Luis. Luis is, therefore, Carlos's nephew, just like Juan. Since Juan and Luis are both nephews of Carlos and their parents are siblings, Juan and Luis are first cousins to each other.

Sofia is married to Eduardo, who is Maria's brother. This makes Sofia Maria's sister-in-law, as the term sister-in-law is used to refer to the spouse of one's sibling.

Isabel is the daughter of Eduardo, who is the brother of Carlos, making Isabel the niece of Carlos. Being the daughter of Eduardo, Isabel is Luis's sister and, in relation to Carlos as her uncle, she also shares the same relationship as Luis does with Carlos.
Carlos has a sister named Maria, and Maria has a son named Juan. Carlos also has a brother named Eduardo, who is married to Sofia. Sofia and Eduardo have two children, a son named Luis and a daughter named Isabel. How are Juan and Luis related to each other, and how is Sofia related to Maria? Additionally, discuss the relationship between Isabel and Carlos. Provide a very brief response.