Hades and Persephone kind of wind up becoming the parents of everyone in the polycule. They're Riah's parents, obviously, Claudine was adopted right out the gate at age seven, and Hades is Mal's father which means, as far as Persephone is concerned, Mal is hers, too. And they're the only good parents anyone really has. Jay's father sucks and his mother vanished, Evie's mother sucks and her father is banished into an interdimensional prison, Mal's mother sucks, Carlos's parents both suck- so they're all in the market for good parents. And so Hades and Persephone, being the only good parents anyone has, wind up becoming adopted by everyone else.
And then it starts expanding. Agony has two very good parents, of course, but Hades has been her honorary uncle her entire life since he's good friends with both her dad and her Uncle Gonzalo so that's how she's been unofficially adopted into his family. Malcolm, obviously he needs new parents, so he gets adopted, too. Ace from Jay's QPR gets adopted, and Rose and Andi get adopted with it since the three are a package deal. Then Ben, whose father sucks, gets adopted. Tavasya, Ace's boyfriend, gets adopted even though he already has two great adoptive parents, and his sister Devika gets adopted since the two are a package deal. And so on, and so forth.
Hades is every bit as much of a mother hen as Facilier is, and he and Persephone have an agreement that any children the one has are automatically both of their children regardless of who the actual other parent is, so Seph is a mother hen by association. Demeter winds up with an astronomical amount of grandkids.