A noun in apposition to another noun will always be in the same case.
ἦν δὲ ἐγγὺς τὸ πάσχα, ἡ ἑορτὴ τῶν Ἰουδαίων. (John 6:4)
λέγει αὐτῳ εἷς ἐκ τῶν μαθητῶν αὐτοῦ Ἀνδρέας ὁ ἀδελφὸς Σίμωνος Πέτρου· (John 6:8)
εορτη and αδελφος are both in apposition to πασχα and ανδρεας respectively.