Complex sentence: Chief Coosa was angry that he was taken so far from his home village, and because de Soto still held his sister, who was probably the mother of his successor as chief in their system of matrilineal descent.
Split into simpler sentences: Chief Coosa was angry that he was taken so far from his home village, and because de Soto still held his sister. She was probably the mother of his successor as chief, according to their system of matrilineal descent.