The Ancient Magus' Bride Episode 17: Look Before You Leap.
Episode 17 of 24 of The Ancient Magus' Bride, aired February 4, 2018, 25 minutes. Available with English subtitles and dub where released.
Episode summary
Stella, a young girl Chise encountered while Christmas shopping, finds that her brother, Ethan, has gone missing and her parents have forgotten he exists.