Aren't fox spirits supposed to gain tails with age and power? How did she already have 9 when she was a little kid?
I think that part was specific to the Japanese folklore (kitsune), while the Chinese fox spirits come in many forms, some of which are nine-tailed, without it indicating a particular age.
Also, remind me what a "second sister" is supposed to be?
From what little I know about traditional Chinese forms of address, I believe it means she is the second-eldest daughter of their parents.