Facts About Halloween's Michael Myers
7 Horrifying Facts About the Ultimate Slasher Killer, Michael Myers
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This is another question that can be answered with "it depends on who you ask." Laurie Strode being Michael's younger sister was a storyline first introduced in Halloween II (1981), and while many of the following sequels stuck to this storyline, 2018's Halloween completely retconned that choice.
Since 2018's film acts as a direct sequel to the original 1978 film, they took the liberty of ignoring or reversing canon from its predecessors. That included the relationship between Laurie and Michael. Halloween (2018) even references the idea that people believed they were siblings by having Laurie's granddaughter, Allyson, converse about it with one of her friends. While explaining the events that led to her grandmother's trauma, one of her friends asks about Laurie being Michael's sister. Allyson replies, "No. That's just a bit that some people made up to make them feel better, I think."
According to the most recent canon, Michael and Laurie have no sibling connection, but their familial connection will probably live on in some capacity through Halloween's earlier remakes.