Half-brother?
Her half-siblings were about ten years old...

Her brother found out and thought she'd gone insane, and then started trying to get her to bet on racehorses.

Luke Perry (who was the guy-trying-to-get-people-to-die) already knew, and was terribly nasty and evil (but still really fun). He's the one I was thinking of.

Davis knew about the whole thing almost as soon as Tru did, but didn't tell her that for a while.
He was, however, always very much on her side...