Eleanor

I missed that, and if it's true, it's probably explained in the book.

I suspect he didn't call her "mom" even though Eleanor was Nick's mother, because she let his grandmother raise him and make him the favorite and heir, and they didn't have a normal kid-mommy relationship. As to why he called her "auntie" and not "mother", well, there could well be some Chinese word for that sort of relationship that doesn't translate well to English, so "auntie" was used.

In a historical novel, I came across description of a 19th century Chinese family, where the father had two wives. The story was about the second wife and her children, and in the culture of those times, the first and primary wife was thought of as the official "mother" of the children, and the second wife and children referred to her as "your/our real mother". The woman who actually gave birth to the children and raised them was known as the children's "auntie", Again, I suspect the word "auntie" is a bad translation of a term meaning "honored elder female relative who isn't my official mother".

updated on post below

In some Asian countries they use Auntie to address older women. If its an older lady waitress or a friends mom then they'd address her as Auntie. It's a friendly way of addressing an older woman. So Rachel addressing Peik Lin's mom as "Mrs Goh" was a real American way of addressing her. She was told to call her Auntie instead, and Rachel thought that would be the right way to address Mrs. Young. Like she was understanding the culture. But you wouldn't come out and call your mother-in-law or future mother-in-law that, so it made her look even more "Westernized" and culturally clueless.