I really can't believe fanboys.
Apparently the centuries old concepts of copyright law, intellectual property, and inheritance mean nothing in the face of fanboy entitlement.
Here's what's going to happen. Either the megacorporation wins outright based on contracts or they write regular substantial checks to the Kirby family after they reach a settlement agreement. And their will be much fanboy gnashing of teeth in the meantime because the fanboys don't realize that it's merely about splitting the revenues fairly rather than outright stopping Marvel from using those characters. And Marvel will get to use those characters, they own Stan Lee's contributions and the trademarks.
Considering how poorly Marvel treated Kirby other than lip service when he lived, it's hard to feel any sympathy for them now. The guy had to fight tooth and nail just to get his original artwork back. And Marvel's reprint fee policy is a joke. Kirby played a big part in making that company what it is today, apparently worth $4 billion, and received a meager fraction in compensation in his lifetime along with many broken promises. There's no reason the Kirby family should look all that kindly at Marvel. Heck, Marvel's had decades to come to some sort of preemptive settlement with the Kirby family, even while Jack was still alive, and did nothing.
As for why the family isn't suing DC. 1. Kirby's 70s creations are another decade out from the window of filing. The copyright isn't close to expiring on those. 2. The Kirby family has a much better relationship with DC which reportedly pays regular reprint fees on his work, even when they don't have to.