As Sean Carroll states here superpositions "come into being" due to "the magic of decoherence". Please criticize the following reasoning: He decides to measure "a spinning particle that can have spin-up or spin-down." He' measures spin-up and he'' measures spin-down. The day before his sister has sent him a letter which will arrive on the next day (after the measurement was performed). Two possibilities: The next day he' and he'' receive the letter. This seems to imply that sister' and sister'' have written the letter which however doesn't make sense as the splitting into different branches is future-oriented The letter was written before he decided to make the measurement. The next day he' or he'' receives the letter. But this seems to make no sense either. The sister exists in his' and in his'' past. So, what's wrong here?