Originally Posted by LyricalAura
I think it was said that Maria bragged to the cousins about the letter after she got it, so at least that part seems to be true.
In that case, I propose the following Blue Truth:
Because the parents are 100% unaccounted for from 7:30 PM of the first day until the next morning, it is possible that the six of them were killed, say, before 11:00 PM. Maria, who wants to check up on her rose, is feeling restless and decides to go check up on it at, say, 11:30 PM. There, she encounters "Beatrice", who hands Maria one last envelope. That envelope has the key.
Edit: Or better yet, perhaps the first note was instructions on when and where to meet up with Beatrice to receive one final gift: the envelope with the key, thereby tricking Rosa into thinking that it was the first envelope all along and convincing everyone that this is the doing of a witch.