Complex sentence: His eldest son, Nanfan, Lord Coloony, succeeded to the earldom on his death, his second son, Richard, succeeded in turn as 3rd earl on his elder brother's death.
Split into simpler sentences: His eldest son, Nanfan, Lord Coloony, succeeded to the earldom on his death. His second son, Richard, succeeded in turn as 3rd earl on his elder brother's death.