Answer:
His sister-in-law is offering him polyandry and the opportunity to keep the family property as one.
Step-by-step explanation:
Fraternal polyandry is commonly found among Tibetan families. Polyandry consists of one woman marrying more than one man. Fraternal polyandry consists of a woman marrying brothers from the same family.
In the scenario described in the passage, the advantage to the younger brother's acceptance of her proposal is that they will be able to keep the whole family herd and land rather than dividing them between the brothers. This creates a large and strong family and property unit.