Tywin might well rebel, but he’d be fighting the Stormlands, the Crownlands, the Reach, the North, and the Riverlands, with his only options for allies being the Greyjoys (who burnt his fleet at anchor) and the Martells (who want him dead). That’s not a fight Tywin can win.
Also, it’s not entirely necessary that Cersei and her kids be executed – Robert would certainly want that, but Renly might suggest instead that they (and Jaime) be held as hostages.
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