In the making of David Gordon Green's Halloween, the decision was made to both embrace and reject certain specific elements of the franchise canon. Namely, while the movie maintains that everything that happened in John Carpenter's original movie still went down, the same cannot be said for the events that played out in any of the other sequels. This obviously means that there is a lot of abandoned material, but certainly one of the most significant missing pieces is the sibling connection between Jamie Lee Curtis' Laurie Strode and Michael Myers.
The most recent Halloween even goes as far as to openly mock the idea, with characters referring to the familial relationship as "something that people made up" - and the derision comes with good reason. The idea to make Laurie and Michael brother and sister was never an approach that John Carpenter liked, and he recently went on the record with ComicBook.com explaining both how it came to be, and his thoughts on it in the aftermath:
Well, the brother reveal was caused by NBC. NBC purchased the rights to show Halloween on network television. But our movie was too short for them. So we needed to add some time. I think we had to add, what was it, eight minutes or something like that, I don't remember. And there was nothing to add. The first movie was just what I wanted to make. I don't have anything to add. So I came up with this brother thing. It was awful, just awful. But, I did it.
It was actually 12 minutes that Carpenter needed to add to the runtime of the original Halloween, helping it fill up a two-hour time slot, and the material was filmmed during the production of Halloween II. One of the included extra sequences featured Donald Pleasance's Dr. Loomis visiting the Michael Myers' cell at Smith's Grove Sanitarium, and discovering the word "sister" scratched into the door.
The sibling element was made firmly a part of the Halloween canon in Halloween II, as the information is included as a twist, but as John Carpenter says, it's not exactly a story element that he is proud of. He never planned to make a sequel to the 1978 original, and it just happened to be a convenient thing to include as a twist for a second chapter when the first became a massive hit.
In retrospect he looks at it as an "awful" plot development, but thankfully it was something that was excised from the narrative when it came time to semi-reboot things with David Gordon Green's 2018 feature.
Now, of course, we're in a brand new era of the Halloween franchise, with two more sequels on the way to create their own new fully-developed branch of the canon. Halloween Kills will be the first to come out, set for October 16, 2020, and it will be followed almost exactly one year later by Halloween Ends on October 15, 2021. Both titles have David Gordon Green attached to direct, and he's also co-writing both of them.
Don't expect any resurrected family ties.
