User blog comment:Gifist8/Will Caitlin Become Killer Frost/@comment-28458080-20160712161422

I think her brother was thought to be dead or he left with Cait's father(we don't know anything about her father maybe he took  the boy and left Cait and her mother alone) then the boy got infected by the particle accelrator and he became Killer Frost but he didn't use his powers then in season 3 he returns and after accidentaly injuring Cait while fighting the Flash he gives her some of his blood then kills himself.