Bob and Helen are out of the superhero business. But when Helen gets an opportunity to be Elastigirl again, she jumps to get back into the workforce. Leaving Bob at home with the now 3 kids including a baby Jack-Jack. Who of course will steal the show, or what is left of it.
Most everything stays the same, the 1960s setting, the demeanor (wife goes back to work, hilarity ensues), the nuclear family. It's not retro now, it is just dated, like the land that time forgot.
This concept is stuck in the past. The cast is old, and the Marvel Cinematic Universe and the Dark Knight have completely changed the superhero narrative. The moral is: Don't wait too long for a sequel.
My advice: rewatch the 2004 original!