*I DO NOT OWN ATTACK ON TITAN*
The Ackerman family has been ruling over the island of Sina ever since time began. The mighty castle has been passed down generation to generation, the empire never falling. Of course the castle has had major modern improvements. Levi Ackerman is the current King who is constantly pressured by his aged parents to get married.
The Jeager family has been ruling over the Rose Ocean surrounding Sina ever since time began. The mer have been worshipping each generation as gods and deities. Eren Jeager is the current emperor who is constantly pressured by his sister Mikasa to attack other oceans.
Oh yeah. Also, the Ackermans and the Jeagers have been ancient enemies since time began. Any Jeager (or any merfolk) caught on land would most likely be sentenced to death. Any Ackerman (or any human) caught in the water would also most likely die a watery death.