Cecile and Papa Justify use their magic to switch their older possessed bodies with younger bodies, constantly 'upgrading' to a new body when the older one starts to wear out. It's never expressly explained, but heavily implied that Luke is actually trapped in Ben Devereaux's body, and Papa Justify switched bodies with him when Ben Deveraux started aging. This explains why the Ben Devereaux body is constantly trying to run away from Violet Devereaux.
The twist in the movie reveals that
Caroline's actual purpose is to be the next body of Cecile, at the end of the movie Cecile succeeds in switching bodies with Caroline, trapping Caroline in Violet Devereaux's aging body.