So I've been reading around the Naruto wikia and came across a statement that Minato's soul couldn't be resurrected (by Orochimaru in part 1) cause his soul is in the Shinigami from when he sealed the Kyuubi into Naruto. Then that means Kushina's there too, right?
They probably waited just to meet Naruto then went to see each other in the shinigami's stomach where they have to fight with the rest who are trapped for eternity.