When they were just kids, Tim shot and killed their father following a fateful night where he'd earlier killed their mother and was menacing them.
In the intervening time, Tim has had to undergo a great deal of therapy but is finally ready to rejoin society, dismissing what he said at the time about events as the product of trauma.
Kaylie meanwhile, on the surface having led a more normal life maintains that their father was under a malign supernatural influence and has been planning to get proof of this as a form of closure. She has a very detailed plan.
To make the plan happen Kaylie needs Tim and reminds him of a pact they made that night. No good will come of this.
CW: injury detail