I'm Emrys Odinson Goddess of thunder

Chapter 1: Why mother?

The current prince and future king of Asgard thor and the current prince of Asgard Loki are training for the battle that might be coming with Jotunheim, it was just another peaceful day in Asgard, Thor, and Loki were riding their horses around the kingdom when they say Odin storming out of the castle with their mother crying running after their father saying:

Frigga: Odin PLEASE

Odin: not another WORD!

Frigga: IS THAT TOO MUCH TO ASK FOR (says while crying) 

Odin: why DONT YOU UNDERSTAND WOMAN I CAN'T FOR THE SAKE OF THE KINGDOM!!

Frigga: I miss my CHILD! (says and then breaks down to the ground on her knees and starts crying)

Odin: FRIGGA! 

he shouts at him and then goes and rides his horse away from the castle.

Thor and Loki couldn't believe what they witnessed, thor didn't really care as much but Loki, on the other hand, was always the one who went to detail in everything, so he very quietly parted from them and went into the palace to Frigga.

Loki: mother, what was all that about?

Frigga: I think it's time I tell you and your brother something call thor.

thor came and they followed their mother to her room

Frigga: This is no easy way to say this but when I was pregnant with thor I had twins, and when thor was born so was your sister... Emrys.
When you were little Loki wasn't born, you were running around in the palace playing with each other until one day lightning struck Asgard, Thor hadn't yet possessed the powers so he wasn't affected by it but Emrys controlled the lightning with her hand and played with it, and at that moment Odin saw the second mode powerful Asgardian or offspring of Odin, she could be even more powerful than Hela.
So then you father sent her to the valkyries, the female servants of the Odin who brought those who were doomed to die in battle to Óðinn's hall Valhǫll in Ásgarðr, because it was an easier way to kill her wasn't it.

Thor: My twin sister?  I have a twin SISTER? is she DEAD?

Frigga: She will be if she goes to a battle with Jotunheim, because yes she might be able to go past them but if they see too much of her power they will take her, bring her into their army, train her stronger than ever before, and make her attack Asgard to either take or destroy the space stone.

Loki: So what should we do about this, we can't just let her die!

Thor: What did she look like?

Loki: What?!

Thor: Did she looks like me?

Loki: Thor we are about to go to war and our sister is about to die, and your worries about what she LOOKS LIKE?

Frigga: You two were identical, you looked exactly the same...

Loki: Oh for heaven's sake of course they looked exactly the same, they are TWINS!

Thor: Where do we find her?

Odin: You are not going ANYWHERE!