Chi wa Mitsu Yori mo Amaku

An older brother and Vampire Aki and his younger half-blood Vampira sister Yura.
These two long-lived creatures of the night have been hiding together in a forest for 80 years.
Aki usually feeds on the beasts of the forest, but he is unable to find food for an extended period of time and Yura decides to let him suck her blood. However, the first time Aki sucks the blood of something other than a beast, he becomes horny with lust and rapes his younger sister.
The two pretend that that it never happened.
But the passion of that night was unforgettable.
Unable to drink any blood, Yura notices that her brother is growing weaker and weaker, and suggests that he bite her again. Saying that she wouldn't mind even if he ended up raping her...
"If I suck your blood again, I will surely do something even worse this time." he replies.
An older brother and younger sister who have supported each other for decades...
Such a gentle, gentle brother he is...
But that night, he bites... and bites again... and cums... and cums again... and again... and again.

(Source: Dlsite)