Complex sentence: Chief Coosa was angry to have been dragged so far from his home village, and because de Soto still refused to release his sister, who in a matrilineal society would have been the mother to the next heir to the throne of Coosa.
Split into simpler sentences: Chief Coosa was angry that he was taken so far from his home village, and that de Soto still held his sister. In their matrilineal society, she was the mother to the next heir to the throne of Coosa.