My research (also posted on 4chan): I googled the original picture's MD5 (which is 85e8eb47be3c3c131d885bb73eabfe08, danbooru's filename ) and it found some japanese 2ch threads that were talking about this image.
In one thread, post 374 asks about this pic amongst others, and someone replies to him: [春輝] ボクだけの妹 (ペンギンセレブ Vol.007) . which means "only my sister (in magazine ペンギンセレブ Volume 007)" Someone else replies "user kEmU on Pixiv", but there weren't any similar images on that pixiv user's profile.
NOTE: The lead above to "ペンギンセレブ (Penguin Celeb) Volume 007" was incorrect (probably to a different image in the referred post 374), ignore it and stop posting it on 4chan.
What makes you think that there is a complete work? I strongly doubt that there's more than these two pics. If it were from a doujinshi, he would've posted the cover somewhere. If it were from a manga, he probably couldn't have posted those pics without permission from the publisher. He doesn't have a Comiket circle profile on Pixiv, so he's probably never published any doujinshi.
But now... even if she's still a kid at heart, at this age her body is already becoming that of a woman...In the past...Y'know, today in school~Onii-chan, what's up...?Is there something wrong with my butt...?*sfx* bechin (loud slapping sound)Sorry...*sfx* goshi goshi (scrubbing sounds)Who the heck would be looking at your butt anyway...Even so, you didn't have to hit me like that!! You stupid!!Ooowwwww~~~~*sfx* biri biri (stinging sensation of pain)I... idiot, hurry up and face forward! How do you expect me to wash your back like that, huh?!At a time like this, "my son", why are you having a reaction... (The word "musuko" normally means "son", but can also be used colloquially as "penis". Obviously, in this context, it refers to the latter.)...um, you know...*sfx* puru (*boing*)accidentally used too much force...Hee... (A quasi-interested affirmation of sorts.)Well, it's true... in the past we used to bathe together... (lit. "we used to enter [the bath] together")She's my little sister you know... Aren't you misunderstanding something here...