What is the fine-grained topic of the following text: Having already frequently performed in the San Francisco Bay Area, it was not until 2001, when UNESCO declared the Kunqu Opera as one of 19 masterpieces of "Oral and Intangible Heritage of Humanity," that the Hou sisters realized that they were torchbearers of a vital tradition. Only return the topic.
Hou sisters' realization of their role in preserving Kunqu Opera after its recognition by UNESCO