
Other mistake: While Nisi and Mickey are in the kitchen, when Isaac and Manley walk in, on the counter is a box of instant grits with tape covering its brand name (Quaker) and a bit of tape covering the face of the company logo. Then when Mr. Blakemore takes out the ice cream there's tape covering its brand name as well.
Answer: He knew right away, because he knew that Lily didn't have any children, so Nici and Mickey could not be Lily's granddaughters.