Otouto no Seiyoku Shori wa, Ane ga Suru Mono da to Onee-chan wa Omotte iru.

My father just remarried, and I now have a stepsister around the same age as me. What's more, is that I'm using her as my own personal fap-pet! She's the shortest at school. She's also got the biggest bust! For some reason, she's under the impression that it's her job to care for my sexual needs, and I'm not about to correct her any time soon~!

(Source: DLsite)

Note: A live-action AV adaptation (MIMK-113) was released on March 17th, 2023.