I’ve been giving this some thought (far more than it actually merits, but that’s what I’m here for) and I realized that I don’t know how Michael knows that Laurie is his sister. She was two years old when he killed Judith, so there’s no way he recognized her (discounting a supernatural connection, which would be a totally valid explanation in this series) at 17. In the intervening time, he clearly learned some things about the world (like how to drive, and what Samhain means) but I think it would be very strange if Dr. Loomis were telling him anything about his family, at least after the first few years of their relationship, given the way that Loomis talks about Michael. So he should have no idea that his parents are dead, or that Laurie was adopted by another family in Haddonfield. In fact, we don’t know for sure that Laurie is even the same name he knew her by. She was adopted at four, but I can imagine the adoptive parents changing her name to try and shield her a bit from the notoriety of her birth family.
So, Michael shows up at his childhood home, ready to finish the job he started fifteen years earlier, but finds it empty, something he probably never even considered. Then, a girl about the same age as his remaining sister would be, who another person calls Laurie within his hearing (assuming this is actually her birth name here), just happens to turn up on the house’s doorstep? I think he decided in that moment that Laurie was his sister, and that he was going to kill her, completely absent any hard evidence to back that conclusion up. He happened to be right, but that’s probably down to Fate or some bullshit, not any actual knowledge that Michael possessed. From there, the only other people he kills in the first movie are canoodling teenagers, which is what (apparently) set him off in the first place, and he uses them to make a shrine to Judith, which makes me think their murders were really just auxiliary crimes, subordinant to his true goal of offing Laurie and making her the centerpiece of his Idol.
In any case, I no longer know whether this plot element makes any sense at all, but I’m pretty sure I need to just move on to the one without Michael, to wipe my brain clean and smooth again.