I'm trying to work out a plot line for a story I'm writing. Who would be the rightful heir to the throne?

King John dies in battle. He and his wife, Queen Anne have no children. His younger brother, James takes the throne. A month later it is discovered that former Queen Anne is pregnant. She gives birth to a boy, and names him Lorenzo. A year or two later King James and his wife have a boy, they name him Aleksandar. Upon King James' death who is the rightful heir to the throne Aleksandar or Lorenzo?