On March 6th, the Imperial Household Agency announced Princess Hanako will be in mourning for 30 days for her older sister Yasuko Nishida (1931 - 2023) who passed away on March 4.
Princess Hanako's meeting with the president of Japan Ikebana Art Association on March 9 at the Hitachi residence will be postponed. Mourning began from the 4th.
Sources:
Asahi,
Sankei
From their father Yoshitaka Tsugaru's Japanese Wikipedia page
津軽義孝, Yasuko was the eldest daughter. She married Ikuhiko Nishida and had 2 sons. The younger son was adopted to be his grandfather's heir (Susumu Tsugaru).