Talking about the letter, it urged  commanded  them to take this baby in and take care of it. And even if it’s someone from  that  world is just having fun and it is just a random baby, not her nephew (who she has never seen, but it’s not the case now). Petunia understands too well the unfair laws of  that world  , which she has never been truly part of, but knows that almost anything can prevent  them from destroying her family. 