Okay so far i have only got this theory for FNAF4 and SL so bare with me as i figure everything out! also the two people who helped me with this theory are 

@hellapeep and my little brother who helped me fill in the holes as well as @GravyOnJello who originally made me start the theory! Thank you all!

In FNAF 4 we all know how in the mini Games we play as a little boy who gets scared by his older brother and later gets killed because of a prank or whatever we also find out they had a sister in FNAF SL. everyone think's we play as the little boy...but what if we play the older brother! Now to back this up ill have to start from the beginning. 

It all started when William Afton (Purple Guy) and his brother Micheal Afton who started by opening a pizzeria where Animotronics play  for children and preform. Micheal Afton being the one who mainly builds and works on the animotronics while his brother William Afton mainly works on the businesses part but they went back and forth. The first animotronics they built were Spring Trap and Golden FredBear. While they were up on stage The boys brother pulls the prank that gets his little brother killed. After that William Afton and Micheal Afton shut down the pizzeria and open up a party business (SL)

Micheal built Baby in honor of his niece and put her on stage not having all the bugs worked out, his niece being completely in love with it walks up to it, Baby bugs out and kills the small child. 

Micheal, having been the one who built the monsters that killed both his Niece and Nephew, kills himself out of guilt in his office leaving William afton to take over SL.

 William now has had both his youngest kids die with in a few months/weeks and his Brother. His oldest son is traumatized that he kissed his younger brother and now suffers from insomnia due to having Nightmares of him reliving that day which can make him super tired and see things. His Father William is worried about his son so he installs cameras in his house to watch his son while he works on the Animotronics down below his house.

William afton after having lost his two children and Brother went slightly crazy and tried to brig back his kids and merge they're souls with the robots. In order to do this he has to experiment meaning he had to kill some children so he built the a compartment in the robots so they could snatch up children to be used in his experiments, And where else to get children other then children's parties.

now this is where it gets trickery for me all i know for sure is that after a while he finds out how to merge children's souls with robots. the children he killed are mad and he gets killed by a suit aka the old spring trap suit and he becomes spring trap.

Now a few things you may be asking.

What about the Pills and IV bag found beside the bed in FNAF4?

Well if The older brother isn't sleeping enough his father may have to put him on medicine to make him sleep and it could cause him to be out for a long amount of time meaning he has to have liquid hence the IV bag.

Why Did he put it (SL) under his house?

Well that would be because the animotronice his is building and or have built are extremeness expensive. so putting them in an under ground storage unit would be very wise.

Now i now i haven't gotten ALLLLLL the bugs worked out in the theory (hehe Robot pun) but just tell me what you guys think so far!