Close and reopen the game after each run that makes it to the Luna fight.

Explanation: The game has a single counter which is set to 0 when the game boots and increments each time you touch a sibling. When the counter changes from 3 to 4 during the fight, it triggers the fight end cutscene. However, this touch counter is never cleared for any reason, even after a save & quit or deleting your file and starting a new run. This causes softlocks when the value is 4 or higher, and also means it is possible to start a run where the counter has previously been set to 3 to make the fight end after touching just one sibling, which is banned.