>Rei is Dawn's ancestor, while Akari is Lucas's ancestor, as already suggested by Khu in one of his tweets
>If you choose to play as Lucas, you'll meet Akari, your great-grandmother. If you choose to play as Dawn, you'll meet Rei, your great-grandfather.
>You'll become friends with your great-grandfather/great-grandmother during the story
>At the end, after you catch Arceus, he'll send you back to the presente, as already suggested by the introduction dialogue
The incest theory is stupid and makes no sense. It's much more simple (and more logical) that Akari/Rei are simply Lucas/Dawn ancestors.

>Why Lucas/Dawn looks identical to Rei/Akari?
Every ancestor so far has been pretty much identical to their present counterpart. Just because Kamado looks like Rowan it doesn't mean Rowan travelled back in time and became his own grandfather.