https://www.dlsite.com/books/work/=/product_id/BJ036236.html
A younger sister NTR.
Or something like that, the ending is more light hearted than you'd expect it from the setting.

紐パン状態のエロ下着を身につけさせられ、「見ないでお兄ちゃん!」
と叫びながら千葉の容赦ない口姦フェラにのたうち、
アヘ顔で自分の乳首をくわえてヨガる妹・千歳。
「お兄ひゃんゴメンナヒャイィ…」と、兄の俺の気持ちを守ろうとする千歳の痴態に、
俺は思わず自分のチ○ポをしごき始めてしまう。
それは強烈すぎる快感だった……。

tl;dr:
A little sister goes out with her brother's best friend so said best friend let the brother watching when he fucks her.

As always, pleas support the author by buying her/his work if you liked it!

An old manga from 2014 by sad.co

Base +8, genico +6

https://e-hentai.org/s/9f94b94aa2/597371-117

down group

Base +76, genico +6