My money is on the Garten family being the villains. They're clearly suspect in all their actions so far.
Instigating war with Konrad for their own gain and poisoning the duchess to destabilize her husband and brother? Seems plausible.
After all the stuff her dad had to go through to win her mom, I expect Xi's parents would be more free to let her make her own choices.
Plus our hero has already a shown himself to be a fanboy of his future father-in-law. That should win him some points.
The character map is useful. I was wondering who Xiao Mo's parent was.
Though the lack of any family of our MC is a little sad since his (great?) grandfather battled Mu Chen in TGR.