User blog comment:MarkManeMcIntyre/Some one fixed Michael. It says he's the second when he is the first./@comment-6506463-20160512051159

I think that was a writer's mistake because by saying Lucifer was the first son, it screws up the whole importance of Sam and Dean in the Apocalypse and their older/younger brother relationships and attitudes made them the vessels for Michael and Lucifer. That or Amara meant that as an Archangel in general, he and his brothers were God's first sons. Maybe Michael was barely older and she didn't know so she called Lucifer older. That would be to big of a retcon to accept.