 In these two cases, it makes sense to think that the person you are using can share your ends, in the sense specified in  §3.1.3.  Your sister-in-law can share the end of your spouse not getting suspicious regarding a surprise party, and, of course, the witness can share the end of Brown’s coming to believe some lie. Perhaps that is why in these cases the person’s inability to consent to your use of her seems to fall short of plausibly implying that you are using her merely as a means.