WMG: Petit Eva

The series is an 'alternate reality' created by Shinji after the events of Episode 22
Remember how Shinji envisioned an alternate reality where he was in the world of a sitcom-like setting toward the end of episode 22? It's likely that Petit Eva is simply something like that, to which Shinji retreated when even his 'normal life' delusion wasn't enough to drown out his traumas.