Noticed that this one shot was also published in Avalon Alter -karma- (girls×garden comics)!
https://www.amazon.co.jp/Avalon-Alter-karma-girls×garden-comics-ebook/dp/B07VT19CM6/ref=sr_1_3?__mk_ja_JP=カタカナ&dchild=1&keywords=avalon+bitter&qid=1623608403&sr=8-3

Any information as to where I could access the one shot that used to be under this anthology? Such as whether it was taken down by a scanlator's request?

The one shot should have been called Tatoeba Konna Shimai (例えばこんな姉妹) by Itsuki Touma (東磨 樹), included in the anthology Anesama Inkou:
(https://www.amazon.co.jp/gp/product/4894219468?ie=UTF8&redirect=true)
(https://book.dmm.co.jp/detail/b216afjm00332/)

Imagine, Akabane introduces Aoba to her mom.
Aoba: Your daughter calls me mom too....!!

P.S. Also in the last panel, I think the name was mixed up, it's supposed to be "Oh dear, she (Akabane-san) is mistaking Aoba-san for her mom!"

TL here,
Just noticed the error; thanks for pointing it out!