Updating my complaint now that I'm re-reading this some years later:
The writing really seems to want to have it both ways with this antagonist in a way it simply fails to actually achieve; framing her both as selfish and sympathetically pathetic, supported by everyone and also abandoned, etc.
There's ways it could have been something like a cool story beat about everyone having their own problems, but the delivery is just too jumbled and forced. The end result of all this circling around the heart of the matter is just that the younger sibling repeatedly attempted murder from simple attention jealousy; that's not something worth any of all this hand-wringing over. She needs jail time and therapy and to be removed from any position of power, full stop.
The king is seemingly also written as a nonsensical hypocrite solely so that the author can get a second duel out of it. Like, what the fuck, he says "we need to honour the result of our duels" and "please do the duel over" in the same breath... Dude, he simply can't be both even-handed and highly preferential; again, you can't just have it both ways!
(I mean maybe it's going to be that some more of the trouble-makers are under mind-magic too, I don't remember this that well, but clearly the author likes mind-control magic a lot. But if so, it needs to appear to be a compelling story in the meantime...!)