https://www.pixiv.net/user/24687966/series/40103

Ironic that I just came across this one on Pixiv last night myself. An interesting setup if it were used as a full manga, too bad its just a handful of shorts.
Also: The witch isn't a lolicon. She wants a successor and only girls can be witches. Both of them are very loyal to her, but neither of them wants to become a girl. The witch tells them she'll change them both into girls and they can decide if its really as bad as they think, decide the successor, etc. Younger brother says he doesn't want to be a girl because he likes how he looks now, so she tells him she'll make sure to retain his facial features. Older brother says he's far too tall and wouldn't look good as a girl, to which the witch tells him she'll make sure height isn't a problem then.

Author also has a TSF story on Comico:
http://www.comico.jp/challenge/articleList.nhn?titleNo=18043

Base +6, wmlsk +7, poopascoopa +6, StoneNeeded +4

double性轉的白子魔女

Base +6, tsurumiumiko +1

为啥我想笑。。

Base +5

I translated it. The TSF story Maea mentioned also appears to be at Pixiv though I haven't read it yet. It's called, "世界はふたりぼっちの神様:"

Base +25