Who Killed My Sister?

I came home from a family trip only to find my sister dead. She has left me mysterious messages as her dying wish.

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To: My Brother
A・ C・ F・ ?・O
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Even in shock, I knew that by solving the riddles I could seek the truth no matter how horrid it may be....

The further I get in solving riddles, the closer I get to the suspect.
But the suspect has a contradicting testimony.



-Will you find the killer?-