[@Bornlucky]
Ah I see, you mixed  2 and 3 to make 4.
Maybe... 2 can involve a couple of native kids, and a  couple of kids native to Sinnoh who moved to Alola. Then the eldest kid of the Sinnoh children goes back to Sinnoh for a few years. During that time, the kids still in Alola bond and grow older. When they're old enough to traditionally get their own pokemon, the other child comes back inflicted with Darkrai's nightmares. ?

Just an idea.