I guess this is the Japanese equivalent of redneck in-breeding. Her brother is also her daddy, and her momma fucked her grandfather (possibly creating her brother) and so it’s just this incessant tangle of recessive genes and rape. But the rule is that you only fuck your own family, and it starts with SURPRISE RAPE on your 11th birthday.
I can’t figure out whether it’s hot or creepy when the mother is helping the brother rape the daughter/sister. Both. Yeah, both.