It's Proof of the End of a Friendship (S01E02)

Iroha remembers that her wish was to cure her younger sister, Ui’s, illness; however, there is no sign of Ui’s existence in her life. She goes to Kamihama City to find clues about her, but is attacked by a Witch and subsequently saved by a trio of Magical Girls, Momoko, Rena, and Kaede. There is a rumor going around about the Friendship Ending Staircase at Rena and Kaede’s school. Momoko and the others believe that a Witch is involved in the rumor, so they’ve been investigating it, but… (Crunchyroll)