Rakuda Tsukai to Ouji no Yoru - Vol. 1 Ch. 4

[MangaDex]

 

[Pooky2011]

Between this and the father/son story, i don't know which is more twisted

 

[AvivaDiva89]

Yikes, that's twisted. It's not so much the incest that's the problem (it's still a problem), it's the willingness to allow someone else to humiliate and physically injure you. That's demented. But I don't think this would seriously exist in real life among siblings unless they both are mentally unwell.