What was the relationship between Fourth Madam Xia and Third Master Xia in Lu Xun's 'Medicine'?

In Lu Xun's novel Medicine, Fourth Madam Xia and Third Master Xia were two important characters. Although they did not have a direct family relationship, their relationship in the novel was complicated and interconnected. Xia San Ye was a wealthy businessman. He was shrewd and good at socializing. He played an important role in the novel. Third Master Xia had once invested in the pharmaceutical factory and provided a lot of help to make the pharmaceutical factory run smoothly. However, in the novel, Third Master Xia was also portrayed as greedy and only cared about his own interests, regardless of the safety of others. In the end, he was used by Fourth Madam Xia, causing the failure of the pharmaceutical factory. Fourth Madam Xia was a woman with a complicated background. She was once the wife of Third Master Xia, but after Third Master Xia went bankrupt, she left Third Master Xia and lived alone. In the novel, Fourth Madam Xia was a kind, intelligent and courageous woman. She used her wisdom and courage to gain an advantage in the competition of the pharmaceutical factory and finally became the manager of the pharmaceutical factory. The relationship between Fourth Madam Xia and Third Master Xia was complicated. They were related to each other but also opposed each other. Although they did not have a direct family relationship, in the novel, their relationship was seen as a symbol of the contradiction and conflict between greed and selfishness, kindness and courage in human nature.