Kinshin Chikan Hakusho

Being molested by your brother is...awesome!
Molestation is a crime.
But if you read this manga, you can experience it!!

Hiro has been molesting his sister who has gone off to get married.
One day, he made a promise to play molestation with a girl who is a lover of molestation on the internet,
But on the same day, he mistakenly molested his sister Chieri, who was on the same train with him, and even penetrated her!
Hiro's conflict begins when he molests and penetrates his sister...

(Source: Comic Mujin, translated)