For a moment there I thought her big sis looked rather familiar, like Shiyu. It was Shiyu instead.
Was she talking about her big sis, or is she just calling Shiyu "big sis" because she's older (this kind of thing happens in plenty of Asian cultures) She's not wearing her date clothes, so this either happens on another day, or before she visited the manga cafe.
That Volume 1 cover though. Honami's holding a book with racy cover art that has the title "♥にぜんぶだして" (♥ ni Zenbu Dashite) which means "Let it All Out in my ♥"
Kokoro (heart). ♥ definitely means heart and not another word being censored, right? Right?