From Japan via Twitter comes “Mysterious Holiday.” A description on the Pixiv page where I found the original:
“One morning, the older brother found himself with women's underwear for some reason. He was changing in front of the mirror as his sister urged him to do so...?
This is a story about a date with a sister who is made to be the older sister of a sassy younger sister by the existence alteration TS.”
Here it is in English using my computer-translation and English writing skills.
Also from pixiv:
“Drawing by Kamori
This story was produced by Amuai Okashi Seisakujo around 2019.”
PDF for Download: