This woman, like her family, is the "center of power" in this novel (2.16.73). We never get narration from her perspective, nor is she in more than a couple chapters, but, man, does she move the pieces in this story.
Shang Su Yi is the daughter of Shang Loong Ma and Wang Lan Yin. She has one brother, Alfred, who is married to her niece Mabel T'sien. Interesting, no? This effectively keeps the Shang and Young money all tied together in three branches. The money is apparently immense. Her family is "so unfathomably rich, it would make your eyes water" (2.16.61).
Her father was "an enormously wealthy banker in Peking" in the 1800s before he moved all his money to Singapore. He subsequently made even more money by controlling "all the shipping lines from the Dutch East Indies to Siam" and helped unite the "early Hokkien banks in the thirties" (2.16.63). This guy knew what he was doing.
It is rumored that his wife, Wang Lan Yin, was the daughter of an opium king back in the day, but this is kept very very secret. Why? Because the Shangs are all about clean bloodlines and keeping up appearances.
Shang Su Yi is about customs, traditions, and ensuring that her fortune doesn't fall into the hands of anyone undeserving in her eyes. Her rules around this are what spur Eleanor to interfere in Nick's relationship with Rachel. To ensure that Nick inherits anything, he must marry well.