The Detective Chapter - Part 2 - Distinction - S1.E17

Satoshi apologizes to Shion (still pretending to be Mion), and asks her to care for Satoko during the Watanagashi Festival. That night, his aunt is murdered; in order to cover for Satoshi, Shion reveals that she is actually Mion's twin sister. After the police questions her, she is brought back to the Sonozaki main estate to answer to her grandmother for all the trouble she has caused.