"The grandfather is not my father."
Translation:Der Großvater ist nicht mein Vater.
Special case of the verb "sein": my father is not an accusative object, but a predicate, which has the same case like the subject, i.e. nominative. See also https://deutsch.lingolia.com/en/grammar/nouns-and-articles/declension/nominative
If you'd tell me the sentence with "meinen", I'd understand "the grandfather doesn't eat my father" :D ..... (isst and ist don't differ phonetically)