This is Chapter 5 in Volume 2 of A Place Further Than the Universe
Megumi was like a big sister to Kimari, whose job was to look after her little sister. She thought it would always be like this, but then Kimari found that envelope. One day, Megumi chats with two other girls about Kimari’s growing relationship with Shirase, and happens to tell them about the money. Later, Kimari tells Megumi about her new plan to join Shirase in a quest to go to Antarctica. Over the next while, Kimari continues to tell Megumi how the plan is progressing, about the excursion to the expedition meet-up, about Yuzuki, about the training camp. Megumi feels their relationship slipping away, leaving her with nothing.
This chapter is based on the first four episodes of the anime.