Looking at the releases from what i can tell there should be a chapter on his sister as well which was published in Kairakuten 2015-01.
Think we will ever get to se it translated?
Once in a while, "Little Brother" can be considered another term for ... wait for it ... "Lucky Bastard" (under breath, GRR!). And THIS fits the bill Just Right.
Hmm, a sequel? Well, Yui does show up, after all.
That ending though...Youth, yes, but being a Hentai Protagonist also gives him essentially unlimited stamina.