My friend had a similar family tradition/superstition (they said something bad would happen if you prepared...) I was really surprised... she was nervous about setting up anything for fear it would cause some terrible disaster... or more accurately, her family would give her no end of grief!
Even if you don't get everything ready... you'll still have the stuff around if something happens... so I don't see why having it ready will make it significantly worse.... plus, why not prepare for the good and assume that nothing bad will happen! (I mean you take precautions to be as safe as possible... but you don't not do things just because there is some risk involved? You don't NOT celebrate a birthday for fear you won't have another... or NOT make reservations for dinner, because you just MIGHT get in a car crash on the way there... etc)
It is up to you, but if it were me I would rather plan for the best and prepare the nursery, hang a few baby clothes and maybe even cook and freeze a few dinners ahead of time. If that feels like too much, you could just sort and launder all the baby stuff and put it back into neat boxes so when you get home all you have to do is pop everything into dressers and onto hangers... but you aren't technically "ready" quite yet.
By the way, my friend with the similar tradition decided to go ahead and get everything ready, and she has a beautiful, healthy, happy, cute little 1 month old despite what her family said!