I applied for, and ultimately received, a partner-sponsored residency visa. Like you, my partner wasn't "included" in the application in the sense that she's already a citizen, and I didn't really understand why they would want information about the family of my NZ citizen partner, especially considering they are all NZ citizens (excepting her father, who has lived there since childhood but remains a proudly stubborn British permanent resident!), but I decided that it was better to include everything, and not give them any possible reason to think that I willfully omitted any information.
If it makes any difference in your thinking, at no point did any one from INZ contact any of my partners family for any reason during the application process.