Iduna's POV:
Elsa doesn't know the truth about her aunt. It's tragic and times where you should forget it. She's just a kid she doesn't know more about her than she could.
"Hiya!" Aganarr was heading back to Arendelle and got Iduna waiting for her. The time he got there Gerda saw him and Gerda is about to fall into lake. she fell there and cannot breathe and cannot go up. Meanwhile Aganarr and Iduna is making up Iduna forgot about her two sisters while Aganarr used his enchanting necklace created by the first Tribal Cult and sang an enchanting song to erase Iduna's memories of her sisters. She fell asleep. "It's time!" he proclaimed his guards to attack those peoples at Arendelle so there could be no peace at all.
A year later she birthed Elsa and 2 years after she birthed Anna. She doesn't remember her sister Ingrid so she wondered why Elsa had Ice powers like her aunt.
Gosh this version is darker it's only on my theory on my mind it didn't really happened or planned by creators!
THE END BTW