Lianyin (Feifei) is the elder sister

Fayin (Fangfang) is the younger sister

This is for sure. The same is true for the official website.

Their birthdays are proof

Lianyin: November 29th

Fayin: December 1st

Kingdom of the Sun

Princess Lianyin (Philippines)

The princess of the Kingdom of the Sun who has not yet been able to stand alone. Sister type. Compared to Faien, she looks girlish and elegant. Belongs to the incredibly generous type. A good dresser who really likes pretty things. But the type that can hold steady when things happen. When Fa Yien starts to hesitate, Lian Yien will think twice before taking bold actions, making things even more confusing. Because he only looks at the distance, his approach to things is different from Faien's.

Princess Faien (Fang)

The princess of the Kingdom of the Sun who has not yet been able to stand alone. Sister type. Like a boy. Activists. Belongs to the type of people who say what they have to say. He is a glutton who likes delicious food. Because he only pays attention to his surroundings, he moves quickly and decisively. She is the type who takes action first, then mixes things up, and rushes forward, but her instinctive wild intuition often makes her hesitate out of fear when things come to a head.