In the FMV game Who Pressed Mute on Uncle Marcus? , the person who actually poisoned Uncle Marcus is Aunty June.

The game has multiple character-focused “endings,” but the canonical solution revealed when you gather the right evidence points to June as the one who wanted Marcus “out of the way” and went through with poisoning him before the family quiz.

So, if you’re asking who really did it in the story, the answer is Aunty June.

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