As soon as I saw The Offering, and heard that Justina had somehow miraculously been spared for years, I knew the game was rigged. I also figured she would swap herself for the other girl, given her sense of guilt.
The big question is, what is the true nature of La Ofrenda: there was some vague mention made of the girls being sent off to be married, but not to whom. My suspicion is they are "married" to a walker in some sort of attempt to appease the gods/ keep the walkers at bay situation. IOW, offered up to the walkers as a sacrifice. This would be in line with the sacrifice of virgins to the dragon in 1981's Dragonslayer, and how the names of some girls were kept off the list.
[size=78%]https://en.wikipedia.org/wiki/Dragonslayer_(1981_film)[/size]