You need to think about this from Xing's perspective and not from our lulz-waifu tainted window. He was invited to a meeting with a pseudo-enemy. His thought of companions are all people who have connections with Madam Nie and all have some part to play with the conversation. Qilin is her daughter and could be used to try to get additional info out of Madam Nie or at least steer the conversation into a certain direction. The Twins are her adoptive daughters and might throw her off with their presence. Bai and Madam Nie have a history, and our cooperation could be used to get into her graces or make her more forthcoming. Nie Wuxing, although he isn't an option to us now, is her husband, and might help to see if the two of them are really as closely allied as they might seem. Xuezi has been plotting with her, so she could be used either as an "in" to maker her more accepting of us, or used to help our plan to steal the manuals. /personalspeculation
Cao'er, as I said, is probably just Cao'er. She is and probably always will be an option for the "pick a companion to cover you" choices based on her stalker-ish nature, her separation issues and the cave thing. As far as what she can offer in helping us, she is a very observant person who might pick up on traps or cues that we might miss. Additionally, she is diminutive and probably would not take much of a part of the conversation. In short, she is helpful and controllable.
Yunzi, on the other hand, would be almost nothing but a disaster. She has no relation to Madam Nie, so she isn't as practical a choice as the others. Also, since we are given the option of meeting Madam Nie as part of the vote, a "B" win would suggest that Xing is genuinely interested in what she has to say, and wants to see the meeting out diplomatically. Yunzi is almost the opposite of Cao'er in that she is uncontrollable and an oddity. We have virtually no control over what she does, as she has shown us time and time again, so she might ruin the conversation by being a brash hothead or at least try to dominate it for her own interests and ruin our chances of learning what we want to know. Also, bringing the Holy Maiden to talk to someone as observant and intelligent as Madam Nie would probably be a terrible idea because we could let slip that we are the Lord, if she doesn't already know that. She doesn't really offer anything that would be advantageous to the meeting, so bringing her doesn't make much sense in that regard.