I must have done something to bug the Learning Scenario, because the plot to kill my brother and steal his kingdom never worked before my scripted death at age 40, despite the plot power being past 100% for ten years. And none of my children were adults. So when I died I had to wait seven in-game years before my heir became an adult.
As the new king, I had very little to do. I didn't feel confident attacking the muslims to the South and I couldn't war against my now-uncle because he was fighting them. So I had to rebuild the plot - which was now near impossible because of the +100 relationship bonus for defending against other religions, meaning none of his vassals hated him enough to want to help me kill him, so I had to wait around with about 40% plot power (I had a lot of Intrigue skill, so a high natural chance). He didn't have an heir so it would still default to me. The guy was old too, so he was probably going to die soon right? At around 61... he was still alive. And now he had a kid, so killing him wouldn't be enough.
I declared war on him because he was no longer fighting the Muslims, and easily won several battles. It was more a matter of chasing his army than anything. And then he died, leaving his two-year old kid on the throne. Who nobody would plot against, and it was hard to get war score up because it was my first game so I didn't know how it worked.
And then when the army was decimated and it became a waiting game, muslims promptly declared Jihad on me despite me not doing anything, and now I have 4000 angry muslims on my doorstep.