The tiyospaye was essential to the continuation of tribal life in Dakota culture... In the nuclear family monogamous marriage was highly virtuous,
although polygyny was occasionally practiced by men who were able to
support more than one wife. If a second wife were taken, she was often
the sister of the first. Or if a man's brother were killed, he felt an
obligation to marry his widowed sister-in-law. He was obligated to marry
the wife of his deceased kola, or brother of ceremonial adoption.
- The Dakota Indian Family
Which conclusion about Plains Indians can be drawn from this excerpt?