William and Henry meet and befriend each other while working in robotics and eventually open Fredbear Family Diner.

Both of them end up marrying and having kids.

Henry has Charlie and William has Michael, Scott (my name for the kid in FNAF 4) and Elizabeth.

William's wife dies and he begins to dote upon Elizabeth.

This opens up the gateway for Michael and Scott's brother rivalry in FNAF 4.

Scott is next to die, being bitten in Fredbear Family Diner.

Michael reforms after this but William still ignores his sons in favor of his daughter.

The Aftons and Henry close the Diner, however, and reopen in Sister Location.

Elizabeth is killed by Baby (who was designed to kill children because William has some Daddy issues)

SL is closed and the FNAF 2 place is opened. William continues to grow unstable while Michael distances himself and tries to be better.

Charlie ends up dying in the FNAF 2 place and Henry finally leaves William over it while Charlie becomes the Puppet.

William proceeds to kill many more kids with his creations (which the Puppet brings back), resulting in a shutdown that leads to FNAF 1.

William is finally defeated by five of his victims and they move on. The others (like Charlie) are still out for blood, though.

Meanwhile, Michael is trying to help the robots over at SL and ends up being scooped.

After that, Michael hunts William all the way to Fazbear Fright (FNAF 3) but fails to kill William/Springtrap

Somehow, Michael reunite with Henry and together, they build FNAF 6, the simulator.

They lure everyone into the simulator and burn it all down.

So chronologically, the games go 4, 2, (5, 1,) 3, 6 with 4 being the brother spat, 2 being what follows that and SL's inception then 5 and 1 occurring at the same time (it's William's killing spree and Michael trying to fix the mess at SL). Then 3 happens after Michael's been scooped and William turned into Springtrap, but the attempt fails and 6 acts a reprise to that wherein (for some reason) using fire again actually works this time.

(Remember when the game wasn't this confusing and it was just some dude name Mike Schmidt looking to make a quick buck at a place where robots come to life at night?)

AN: This is just what I think the FNAF timeline looks like. Feel free to correct me because it's really confusing. Also, I'll say right now that some of these bullets are headcanons and others are assumptions. I didn't have the motivation to scour every game for every date in order to get the timeline just right. This is only a timeline theory. Once again, feel free to correct me and call me out on errors.