This helped me piece things together.
Your diagram does not keep in mind the door to the deck.
Thanks. That was an embarrassing omission and potentially a game changer.
I realized that the op mentions FOUR children with the flu bug (not actual flu). We know two of them are the under 18 month olds upstairs (possibly d’s kids or Doucher’s or both). Presumably the third is the foster kid going ape crap. I now suspect the fourth is another foster kid of SIL’s (op says “foster KIDS”). However, that kid is unaccounted for in this story. Perhaps he or she made an exit through the conveniently placed deck door and could not be found by SIL, putting her in a highly agitated state leading up to the incident.
It’s just one theory, but I think it accounts for all of the loose ends.
[attachment deleted by admin]