Great question! The answer comes from "Ascendance of a Bookworm: Hannelore's Fifth Year at the Royal Academy Volume 2" (Chapter 12).

Who is "Big Sister Hannelore"?

Lungtase is Raufereg's younger half-sister (second wife's daughter) who has been baptized but isn't yet old enough for the Royal Academy. After Hannelore agrees to protect her, Lungtase sends a message:

"Codehantz presented one more board: 'As I am now under your protection, Lady Hannelore, may I address you as my big sister?'"

"Me? A big sister? At once, my thoughts turned to Lady Rozemyne and Lady Charlotte. They were surprisingly close... Lungtase and I weren't sisters of the same mother, but perhaps we could get along just as well." —from chapter body10.xhtml

Why "big sister"?

It's a deeply personal term of endearment and respect. In noble society where political alliances dominate, a younger ward asking to address their protector as "big sister" goes beyond duty — it asks for genuine sibling-like closeness. Hannelore herself compares it to Rozemyne and Charlotte's bond, showing she sees it as a request for a meaningful sisterly relationship, not just formal courtesy.