Alejandra is married to Carlos, and they have a son named Javier. Alejandra's brother, Diego, is married to Lucia, who is Carlos's only sister. How is Lucia related to Javier, and what is Diego's relationship to Carlos?

Provide a very short response.
Lucia, as the wife of Diego and the sister of Carlos, is Javier's aunt by marriage to his uncle and by blood as his father's sister. That means Javier is Lucia's nephew. Diego, being married to Lucia (Carlos's sister), is Carlos's brother-in-law. Diego is not related to Carlos by blood, but by marrying Carlos's sister, he has become part of the family as a brother-in-law.