Why doesn't she just have the nuns kill her harem? I feel like that'll solve all of her problems. The only iffy one would be her younger sister.
Well for starters, the nuns are based on a fanatic cult, and their only advantage is the sheer number they have to overwhelm Marie and Lafine over here.
Now let’s look at the harem members so far.
We have the secretary, who is a confirmed yandere from the start, willing to start a fight of knives just to make Marie hers, and during the Lafine conquering chapters, she even distrusted Marie and refused to pick up the slack to investigate, making Marie less of an authority here with not much power over her, making her even harder to let down her guard and be killed anyway.
We have the two maids who didn’t get their own chapter, but they serve Marie daily and diligently, and seem to be the most harmless as well. Extremely low-hanging fruits, if you will. But taking down two at the same time can still be a challenge for Marie, ot to mention they are also directly responsible for her execution in the first place, so you don’t know how much influence they have.
Then there’s Yuuki the swordswomen, aside from being able to slice Marie’s head in one move, she is also an absolute ladie’s woman from the moment she was introduced. If she’s dead, her admirers will come for her head instead.
The little sister always have her masochist bodyguard at her side who seems to be capable of maneuvering without others noticing, and the little sister herself is adored by literally everyone in the kingdom. so killing her will just get her ass executed, either by her bodyguard, or by the fanatic citizens.
As for Lafine, like a few others before, she‘s the goddess of the poor people so she has influence over a wild crowd, plus she is the mentor of the current executor who will be outraged and eager to find the truth if Latino ends up dead.
On top of that, even though Marie can dumb them down and keep them at bay with sex, if one harem of the member is suddenly dead, the others will be suspicious. All she is ever able to do is execute people with orders, not carrying them out herself, so killing either directly or indirectly will make her the target of suspicion. And if one of them is not loved anymore, the others will be doubtful of her sincerity, which is what kept them here in the first place, with perhaps the little sister and her bodyguard as the sole exception because they’re just freaky like that, the others’ conquering process has some degree of love that made them believe Marie genuinely cares. (That, and Marie being the ever frivolous scummy lady, actually wanted to stay alive by improving her image, so less women would want her dead, one way or another.)
Plus, actual killing as a catfight method is not so appealing from the perspect of a harem master anyway. Lesbian Sex is much better.