When speaking to smart people, one needed not to be too explicit in his words. It would suffice to make his point known.
"Qiao Nan, what do you think about this matter? If I wish to have another elder sister, will you allow it? After all, the elder sister is yours. If you don't agree, don't worry, I'll definitely not fight with you or be too close with Sister Qiao Zijin. I'm a good person and very nice to talk to."
Qiao Nan, your decision if you would.
Choose an outcome that would be good for you, me, and everyone. Or would you rather fight with me for the sake of an unrelated person?