Carlos has a daughter named Sofia and a son named Eduardo. Carlos's wife, Maria, has a sister named Isabel. Isabel is married to Paolo and they have two children named Lucia and Marco. How is Lucia related to Maria and Eduardo, respectively, and is it correct to call Sofia and Marco siblings?

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Lucia is the daughter of Isabel, who is Maria's sister. This makes Lucia Maria's niece. In relation to Eduardo, since Isabel is his aunt (sister of his mother Maria), Lucia is also Eduardo's cousin.

Sofia and Marco are not siblings. Sofia is Carlos and Maria's daughter, while Marco is the son of Isabel and Paolo. Since their parents are siblings (Maria and Isabel are sisters), this makes Sofia and Marco first cousins, not siblings.