Well, in Henrietta's favour she does have an Evil Pedigree that is comparable to Louise, it could be her natural blood inclination to start striking out on her own. I mean look at her ancestors, most of them were contemporaries to the Valliere Villains and they still managed to be Royalty and hang on to it, probably even before the Valieres ever became royalists. There must be something there that has to give them the special push that has them ruling Tristania for generations, have their seat of power right above an access to the abyss and all those lovely, lovely relics.
Then the whole comment on the power comparison between Vallieres and the Royal family, Devastation vs Necromancy. So yeah, Henrietta is lower level but she does have some natural bonuses that could make up for that, not to the level of Louise's Evil Chosen One bonus but still notable enough.