The only choice you need to make to become Spartania when playing as Elysia is after getting the event to send your brother on an expedition (or the second time if you postponed it on the first event)

A year later an event will come up to let you join him. The text will be red from memory and doing so will make you become Spartania and Elysia will be taken over by the AI.

Thanks Alot! i was trying to do this in Multi player and i got confonsed