I am pretty sure they won't be a sequel because the main characters are exactly the same as Sun and Moon ones, with different clothing and hairstyles, that also happen to live in the same house. Now, as some others have said, it might imply in this alternate version of the story we've either always lived in Alola or have lived there for a good while at least before the start of the game. And while I think they're going for some kind of alternate universe route, sort of like SM after all, I doubt they will drastically change other characters (aside from purely cosmetic changes) making them evil instead of good and vice versa or whatnot (and surely no gender-swapping). That would be weird to handle with the anime, and in future titles as well.
Not sequels but I think they'll heavily change the story, more compared than usual third versions, while also expanding and changing the region. Maybe they could have a similar ending though, just so that they could give us a story continuation on the same path as the originals, in the post game. But their overall direction should become clearer pretty soon once we start getting more information, so for now it's a bit early.
I do also think they'll show us a free Po Town, and other sorts of changes. Hopefully the Lake of the Sunne/Moone will have more relevance as well this time, that was perhaps my only disappointment with the region.
As a side I can't believe some people don't like the following Pokémon feature, to the point they don't even want it as an option. Way to ruin the fun for others too, lol