Pixiv: https://www.pixiv.net/user/889287/series/48599

(The black haired girl used to be his big brother.)

There's over 18 parts, and I'm unlikely to translate more than the beginning. All that matters is that it exists, and later their relationship becomes very lewd:

https://www.pixiv.net/member_illust.php?mode=medium&illust_id=73383178

I guess they'll swap bodies in ch 17 and there's a related short story:
https://www.pixiv.net/novel/show.php?id=11164809
https://www.pixiv.net/member_illust.php?mode=medium&illust_id=74866783

I guess there's also more lewd pages in the Fanbox.

This looks very interesting. Pixiv can be a pain to navigate, it would be great if someone made translated gallery containing whole story. No pressure though, thanks for showing us this artist.

Base +8, hitmeup +6, Ildzmer +7, somercet +10

I thought he was being insensitive about his bro having a girl over.

But no, big bro WAS the girl /pikachu face

Base +6, Soldier A +6, Ildzmer +7, chesttumors -6, somercet +10

Wasn't it more like Aniki (big bro) in a friend like manner, like a respectful version of calling someone older than you, like the ones you see in delinquent school animes. Since this looks like the continuation of one of his doujins.

Base +9, Jalis +7, Jowling_K +13, MaulanaMuhammad +6

the artist DOES do M>F transformation stuff, so its likely that like the uploader said

Base +5, somercet +11

这个胸反重力了喂！画的时候希望考虑一下。
The gravities for chest is opposite in the last picture.

Base +6, somercet +11