The Corleone Family already had a stake in Nevada before Michael assumed the Donship. Remember when Michael met with Moe Green, he said that the family gave him a big chunk of money to finish his hotel. I think that even if Michael had been killed in Sicily or never got involved in the family business, either Vito or Sonny or both would have expanded their interests in Nevada. What pushed Michael over the edge--that is, to relocate the entire family operation to Nevada--was his massacre of all the other Dons in NY in 1955. He then was able to leave the "olive oil business" to an ally (Clemenza) and attempt to "go legit."