Alejandro is the love child of Camila and Padre Reyes.
As Camila was young, unmarried, and separated from her lover due to his vocation as priest, she decided to give him up for adoption and he grew up in an orphanage. Camila almost addopted him years later, when she was pregnant with Alma, but decided against it on advice of Geraldine.
When he was an adult, he reached out to Camila and they stayed in contact for years without the rest of Camila's family knowing about him. He eventually had enough of being treated like a shameful secret by Camila.
However, Jacob, Alma and Becca changed the timeline, resulting in Camila and Jacob adopting him as a child, and he grew up with Alma and Becca.