Seriously, MC, I think the answer is that almost all the men in Corleone were dead, not necessarily in Sicily has a whole.

From the novel:

"The province held the town of Corleone...but there were no longer any of the Don's relatives alive...All the men had been killed in vendettas or had (also) emigrated... He was to learn later that this small poverty-stricken town had the highest murder rate of any place in the world " (italics mine)

Appolonia was from a different village; remember Mike, Calo, and Fabrizzio had hiked a considerable distance before Mike was struck by the thunderbolt. Their hike took them from the villa where Mike was staying, through Corleone, and then to the village where Apple lived.

The novel also makes reference to Mike renting a car the following week to go see her.

The wedding, I believe, took place in her village.