Memory, Belief.
The way that the Land of the Dead works pretty simply all things considered. It's run by memories and belief. Belief, unfortunately, beliefs can be vague and varied, and misunderstood, and have consequences that no one could expect. A young child believes many things after all, and a simple misunderstanding can warp into consequences nobody could expect.
It's innocent enough, except that it's not innocent at all, and one of the worst crimes that you could commit still has consequences and fallout. Just ones that nobody would have expected while still alive, and even when dead, it's incredibly rare.
Poor Hector certainly didn't sign up for pregnancy, but he has to deal with it, as do the rest of the Riveras.
And Ernesto, the one who caused it... He's just going to sit under that bell, where he belongs.
You don't know how or why, but you've been isekai'd into the world of Jujutsu Kaisen. Although your first instinct is to stay far away from the plot, you've been blessed with an abnormal amount of cursed energy, and for better or worse, you find yourself sucked into the storyline. You decide that you may as well use your newfound powers for the greater good, and if you're lucky, you might succeed in rewriting some of the characters' fates. But it turns out that your presence in this world is even bigger deal than you first thought, and soon, everyone wants to make you theirs.