Javier is married to Isabel, and they have a daughter, Sofia. Isabel's brother, Carlos, is married to Lucia, and they have a son named Marco and a daughter named Elena. Lucia's sister, Teresa, is married to Alejandro, and they have no children. How is Alejandro related to Sofia, and what is Marco's relationship to Teresa?

Make your response very short.
Alejandro is married to Teresa, who is Lucia's sister. Lucia, being Isabel's sister-in-law (as she is married to Isabel's brother, Carlos), makes Teresa also Sofia's aunt by marriage because Teresa is her mother's sibling's spouse's sister. Therefore, Alejandro, as the husband of Teresa, would be Sofia's uncle by marriage.
Marco is the son of Carlos and Lucia. Carlos is Isabel's brother, making him Sofia's uncle. Since Teresa is Lucia's sister, Marco is Teresa's nephew.