Why is Jet Li's version of Huang Feihong Lee Rosamund Kwan called Aunt Thirteen?

Because Rosamund Kwan's role in that movie is thirteen aunts, just as Jet Li's role is Huang Feihong. 13th Aunt is Huang Feihong's mother's sister, and she ranks 13th, so Huang Feihong calls her 13th Aunt. But she and Huang Feihong's mother are not sisters, so they are not related by blood, and they are the same age as Huang Feihong, and they like each other. In fact, they are boyfriend and girlfriend with Huang Feihong.