Complex sentence: They offered to shelter the Susquehannock, distant relations through the Iroquoian-language family, and sometimes allies; though in the Amerind way, sometimes rivals and enemies--as recently as ten years before the Susquehanna had thoroughly trounced the Seneca and Cayuga.
Split into simpler sentences: They offered to shelter the Susquehannock, distant relations through the Iroquoian-language family, and sometimes allies. They had also been rivals and enemies--as recently as ten years before, the Susquehanna had thoroughly trounced the Seneca and Cayuga.