You'd think the emotional scar the Duchess left on the father would be too much for him to even dream of going against the Salvatores.
I, also, find it ridiculous that the sister thinks, even for a second, that her stolen power would be a match for them. She must have her head so shoved up her own ass to believe they will just throw up their hands and shrug it off should they succeed in killing, let alone harming, Leslie.
The kingdom, no matter who they are aligned with, wouldn't be able to save them. It doesn't take a genius to understand they are, quite literally, playing with fire.