Only mentioned once or twice, I think. Rin used her as the basis for her 'imouto' persona.
Edit:
Found it, first mentioned in Chapter 16. https://mangadex.org/chapter/44c32c3c-12b3-4256-9d4a-1e7f62ff291d. There are also other chapters where:
(1) Eiji was chaperoning Rin's sister and her...