Sociopath Community

This one is way too easy. He murdered his sister hoping that the girl would show again at the funeral. Maybe the girl was a friend of the family. If she came to the first funeral, maybe she'll come to the second.

Come to think of it, he could have killed her out of self defense. We don't know how his mother died.

Back home, he asked his sister about that girl's identity. His sister told him the girl was her secret bisexual girlfriend. He got jealous and also wanted to eliminate competition, so he killed the sister. He also got the opportunity to mourn his sister in the arms of the girl, while silently enjoying the feeling of her breasts against his face.

Well, someone said we're free to nuke this thread, so...

(Shhh... You're making them see through the cracks in my mask. XD)

It's interesting that you mention breasts specifically..

That's almost the same as:

A young family, mother, father and one child. 
One day the father dies in an accident, the funeral is attended by one of his colleagues. 
The widow falls for him immediately.

One week later, the child is found dead, killed. 
Who was it and why?

I don't get it. He killed his sister cause she didn't give him the chick's ph#? He had a temper tantrum? Meh, who cares