People grieve in different ways
Our tale begins in the Dumbledore household shortly after Ariana's death. Albus' phoenix Fawkes who also happened to date his sister isn't too happy with him.
Albus admonished, "Stop staring, it's rude."
Fawkes scowled. "Oh, I'm so sorry that I'm staring. I want to make sure that I remember every single detail of the face of my girlfriend's murderer."
Albus asked, "Do you really believe that it was me who killed her?"
Fawkes said, "I was there, Albus. I saw the spell that killed her. It came from your wand not Gellert's."
Albus frowned. "If that's true, then I'm sorry. I, no, we didn't mean to hurt Ari. Especially not me, Fawkes. I'm her older brother, it was my job to protect her, but I couldn't even do that right in the end."
Fawkes sighed. "I'm sorry too. People grieve in different ways. My way is blaming someone else. She told me that it would be an accident and it would be no one's fault, but I didn't think what she said would actually cone true. Maybe Ari was magical after all, I saw her sleeping. It was like she was dreaming, but she floated, Albus."
Albus murmured, "It doesn't matter now because she's gone. There's nothing that anybody can do to fix that."