In the past twenty-four hours, she's been forced to have sex with bullies who raped her. She's been forced to have sex with someone who was also being forced. She's had sex willingly, but it was for the purpose of making the further forced sex that's coming her way tomorrow less arduous to endure. And given the artist's commentary about "deflowering", these are probably her first sexual experiences.
This is her chance to have sex with someone who cares for her deeply, and whom she cares for deeply, for the purpose of showing and sharing love.
Yes, it would be much better if it wasn't her brother. But right now, her choices seem restricted to: having some loving sex with her brother, or letting all the sex she's ever had in her young life be that which was directly or indirectly pushed upon her by rapists.
Seeking comfort, even in the wrong or harmful manner, is ingrained in the human condition. Don't believe me? Talk to someone that is a recovering addict. Misery loves company. The fact that they are brother and sister and just went through a traumatic-as-hell event and are able to "do it on their terms" is also probably helping them, in some small fucked up way, to get over what happened to them in some sort of reclamation/defiance to their abusers.
Seriously, this is simple human behavior. Not even psychology level shit. You think it's ultra fucked up (and, don't get me wrong, IT IS), but I'm also guessing you weren't (gang)raped and beaten and forced to rape your sibling. These two have. Their mindset would be from someone suffering mental (and physical) trauma. Yours is from reading a piece of media.
Also keep in mind that there are... sensitive... "cultural issues" coming from Japan where even if you were a VICTIM, there is some shame that is placed upon you. After seeing some cases of foreigners (especially English teachers) in Japan getting a taste of the wonders of the Japanese legal system (one account of a female English teacher from America that was accused of shoplifting... despite not having left the store and not attempting to hide or run away from paying, no video evidence backing up the charge and was kept in prison for months and forced to sign a confession - to add insult to injury she lost her job and almost got deported and later the security guard actually wrote her an APOLOGY LETTER saying how they knew she wasn't trying to shoplift but their boss was pressuring them and they had a quota(of what, I always wondered?) to meet. Yeah. And that was for a VICTIM.)
Just sayin'. If I was a female rape victim, especially one where I was forced to rape my sibling and it was on video... I would be apprehensive about coming forward and this is in 2019 "HashtagMeToo" and "BELIEVE WOMEN!" America. In Japan? Man... I dunno.