This is important to read through for the story to make sense.
Percy has a twin sister, Katherine (Kit)
Jason also has a twin sister, Valerie.
Even though Kit is a daughter of Poseidon she trained as a medic and healer.
The two sets of twins were switched by Hera for 8 months
Kit fell into Tartarus with percabeth
Takes lave after Blood of Olympus and during Order of the phoenix
ToA is not relevant
Leo is back
Story focuses a lot on PTSD and mental health
Spoilers ahead for those who haven't read House of Hades or the Order of the Phoenix