Complex sentence: Her brother, who would later become her principal record producer, A.B. Quintanilla III wrote two songs, their father and manager, Abraham Quintanilla, Jr. wrote an additional two tracks.
Split into simpler sentences: Her brother, who would later become her principal record producer, A.B. Quintanilla III, wrote two songs. Their father and manager, Abraham Quintanilla, Jr., wrote an additional two tracks.