If an interview is required, the Beijing offices have been requesting only the PR applicant needs to be there, and saying that the sponsor should be available for a possible phone call. (which I did not receive)
You can see if you in the queue for an interview by requesting the GCMS notes (http://cectoolkit.weebly.com/faqs/what-does-gcms-stand-for-how-do-i-apply) - takes 30 days to receive.. or calling your MP and having them inquire, or (as we did,) fax the Beijing Embassy and ask directly. They emailed us both with a notice that we were "in the waiting queue for an interview" 2-3 days after we sent the fax (in English of course) from Hebei.