Halloween and Halloween 2 are both set on the same night of October 31, 1978, one right after the other. According to Halloween lore, the protagonist of these two films, Laurie Strode (Jamie Lee Curtis), marries a guy in 1980, two years after what happened in Haddonfield.

Skipping the third film, which has nothing to do with the Michael Myers storyline, Halloween 4 is set 10 years later on Halloween 1988. At this time, we are introduced to Laurie’s daughter, Jamie Lloyd (Danielle Harris), who was adopted by the Carruthers family after Laurie and her husband die in a car accident approximately one year prior. 

At some point during Halloween 4, the Carruthers' daughter Rachel (Ellie Cornell) makes a comment about Jamie being seven years old meaning she was born in or around 1981. 

In Halloween: H2O, which is widely established as part of the storyline set by the previous six films, we are introduced to the fact that Laurie Strode faked her death and has been living in California and working at an elite prep school under the name Keri Tate. Among her students is her son John Tate (Josh Hartnett) who just a few months prior to Halloween 1998, when this movie is set, celebrated his 17th birthday (he receives a birthday card from his estranged dad that day that is "only a few months late"). This means he was born in or around 1981 as well.

Given the pair of “in or around 1981” births, John and Jamie are, at the least, Irish twins and, at the most, actual twin siblings.

So why don't they know about each other? Why would Laurie choose to save one and not the other? If Laurie didn't "die" until about 1987, you'd think they would have memories of each other. They were approximately six years old when they were separated.

I’ve read a few wikis and other articles trying to get an answer but no dice. This article tries to say that H2O and 4/5 are different branches of the same story but other articles I’ve read say they’re one and the same. 

So what gives???