Shu Gabi is the mastermind, isn’t she?

They certainly are trying to point that way, but it could also be a red herring. Either way, that double suicide does feel suspicious without more context

Oh, what an interesting idea. This is the first episode where I’ve really started to go “huh… things aren’t adding up.” Like sure, there were a few minor oddities before but this one practically had neon “LOOK HERE” signs. However, if I had to point the finger at anyone, it would be someone from the Kou clan. Shu doesn’t seem to have a good motive. But if it was Kou, their motive could be to get Reirin a healthy body.

Your “it was murder” take is actually solid. The mother’s abusive attitude doesn’t give me “suicide” vibes, and a double-suicide seems even less likely. Adding to the “staged” nature of the parents’ deaths, it’s kind of interesting that Keigetsu just happened to be locked in a room with books about forbidden magic. I don’t think those are the kind of things you’d just find in the back room of a commoner’s house.

The other big red flag I saw this episode was the incense in sick Shu Keigetsu’s room. Keigetsu was too sick to light it herself, it was there before the empress and prince arrived, and it clearly had some kind of dark aura about it. It also broke when Reirin reached out to Keigetsu after hitting the bullseye. The only people who could have had access to Keigetsu’s room are people of the Kou clan.

Also how they keep portraying Keigetsu as a tragic character, with Reirin constantly trying to help her and actually happy they traded places because she can be healthy for once. Really makes me think that the titular “Inept Villainess” is actually Keigetsu.