Aunt adopted Mei after miscarriage, but couldn't cope with loss and cept making dolls with one looking like Mei that she placed in... coffin? Yeah she didn't stop slipping into madness :/
So it's not Kouichi fault to start the curse, since Mei sister Misaki died due leukaemia. But that's not tragic enough to account for the curse, others died in more gruesome way...
Mei holding back too crucial information, not only the supposed curse starting that would make Kouichi innocent but also able to see the "dead one" by not seeing their colours. But it's likely they would believe...