Michael never had a motive for what he did in Halloween (and I don't like the brother/sister reveal either though I confess I wasn't aware of it until I watched Halloween H20, since I'd never got around to seeing Halloween II and if the other sequels mentioned it, I'd forgotten it), but what he did didn't really change, from one movie to the next; He went around killing adolescent on Halloween. Having him randomly harm young kids isn't really going back to his original approach so much as it's fundamentally altering it.
I'm delighted the reviews were good (and I agree that most of the sequels were bad) but frankly I don't care. This just feels... wrong for a Michael Myers story.