I dont remember much except the protagonists were a high school guy and his sister which is an adult.

If i dont remember wrong, some sobrenatural stuff happens, and they get a number mark on their bodies that makes them participants and targets in a competition. Now they have to find other participants and kill them to win.

I remember, that the sister seemed a normal person initially, but she was obssesed with his brother at the point of yanderism.

There was a scene where a guy, which is part of the game threats her to kill her and her brother unless she has sex with him. She manages to distract him for a moment, wanting to keep her brother safe, when suddenly a girl with a higher number (which marks her as stronger) appears from nowhere and kills the guy to absorbs his power or something like that.

The girl leaves the woman alone, though she seemed prepared to kill her if she tried to start a fight.

Some of you knows the name?

Sounds like Re:Birth - The Lunatic Taker.

[img]http://www.dragonflycave.com/wpay/mew.gif[/img]