I think your thought isn't so random; in fact, some things that have come up in the story so far seem to show that Luceris is more in dark about what may really be going on than what he has the capacity to realize right now.
First, let's take a look at Luceris himself. Can we really say he's doing any of this with a clear mind? He was more than willing to send MC to the afterlife with their parents had Lancelot hadn't painted the fact that the person he was about to slaughter was younger than his own son. His goals are driven by his grieving love for a lost wife and a need to fill the hole of the loss by pursuing vengeance against anyone connected to her supposed murderer, no matter how uninvolved they were in the possible plot (like child MC). The moment he was given a name to target, he likely went charging for it and didn't give the clues presented to him the thorough look over needed to spot if something is off. And the years will likely not lessen his emotionlly cws.
Now, let's look at the evidence he presents MC of the sister's guilt as a murder culprit: a plant that is native to the homeland, a maid accomplice witness, and a letter apparently written by the sister. There's no denying all of this is quite incriminating, especially when one also adds the apparent history she has with him. However, all this evidence is not as intangible as he makes it seem; these are things that can be tampered with. And such a obvious path leading to said culprit (one he was strongly inclined to suspect in the first place), it makes the idea she is the true mastermind even more questionable: after all, someone who worked slowly and secretly to gradually poison the queen doesn't sound like a person who would leave behind such a obvious trail...unless they were leaving that trail behind on purpose to misled. And what could be more of a perfect scapegoat to their crimes than someone the king would easily suspect in his emotional state?
Finally, there's the initial rejection path and what it gives players a peek at. I feel it strongly implies that there may be a third party involved in all this, and one that is actively targeting MC and maybe even Luceris or/and Helios as well.
Now all this doesn't completely mean the sister wasn't some part of what happened to the queen; unfortunately there's currently no real solid evidence right now that proves someone else might be involved or that the current evidence has something off about it. But there are bits here and there so far that bring to question just how much can we really trust things are really as Luceris and his subjects make it seem.