If you just want the using that audio, it's often a Niconico medley or a speed edit featuring Ichirou's line. Let me know, and I can help track down a link or describe the most likely one you're thinking of.
Conclusion / Quick copy-paste options
( Uchi no otouto maji de dekain dakedo mi ni konai? ) → "My little brother is seriously huge, won't you come see him?" uchi no otouto maji de dekain dakedo mi ni kona link
It falls under the Hentai category, specifically focusing on "Onee-shota" (older sister and younger boy) dynamics. Availability and "Links" If you just want the using that audio,