Can I be so real as someone who genuinely values you and your company?
You're not at fault or wrong for being disgusted and weirded out by a joke like that, especially since it dips into your personal trauma. You being hurt and upset by those kinds of remarks, especially just being thrown around, is absolutely correct and you're entitled to your own feelings.
However, rallying the crowd like this person genuinely did something wrong isn't right. You are allowed to be uncomfortable without having a villain in the equation, but what you're not allowed to do is throw the word "pedophile" around like it means nothing. It's dangerous. It can hurt people in their real life. Nothing pedophilic happened, there were no children in the original context of the joke.
Unfortunately, incest is in the source material of the fandom. Not saying it's cool or okay, but what I am saying is this is exactly the kind of stuff to expect from an account like that. It's not crossing a line, it's not going too far, it's simply a matter of someone having and playing in limits that are farther from your own.
Lastly I want to add that the joke itself is so, ridiculously out there that none of it was meant to be taken seriously... at all. The joke isn't the fact that they raped and killed their child, the joke is the sheer absurdity of the event. It's an adult who crawled out of a muses belly-button and fell directly into the most brutal, grotesque form of torture delivered by the other muse because they dared to be alive. Do I find it funny? No, it's not really funny to me. It's just gross, edgy, "shock-value" type humor. But that doesn't make them, the people behind the RP accounts, incestuous, pedophilic monsters.
Let's not forget these are characters they're playing. It was an off-color joke that struck you, and again, I AM NOT BLAMING YOU FOR BEING UPSET, nor am I calling them 100% in the right for making such a wrong joke, but name-dropping them and calling them all these incredibly dangerous and over-the-top names over a joke is... not it.
I hope you're able to distance yourself from this, block everyone who bothers you, and recover swiftly and safely. I do care about you and your well-being, and there's no one in the right or in the wrong here. This just seems very emotionally charged and I don't want everyone to explode over an impulse, on EITHER side.