Good News! I can make the Halloween Movies make sense!
Bad News: I had to come up with this "Fan Theory" in order to make sense out of the Halloween Movies.
None of it ever happened, Laurie Strode knows that she is adopted and made everything up in order to cope with not knowing who her biological family is.
Why would she think that her biological brother is a serial killer who wants to murder her and anyone who stands in his way? Michael Myers represents the truth of her existence and she feels threatened by it.
She keeps making up new versions of her story based on what is going on in her life.
So Laurie Strode is actually the crazy one.