Ok...
I will add more to my theory.
First, let give another nickname to the miyus
The childhood miyu (Or miyu C)
The other miyu (miyu O)
Ok, that said...
The one that died was not the Miyu C, but Miyu O.
Like this Miyu said, Miyu O got the letter instead of Miyu C. I think this caused Miyu O death and Miyu C had a mental breakdown.
I mean, even the news article fits that theory, both Miyus have the same name, so even if the other miyu died the article would be true.