To clarify:
Elsie was created before Sunny’s canon mom was established;
I headcanon Sunny and Izzy as sisters ever since the movie
There were many reasons for this, mainly:
Sunny representing the Sun (SUNny) and by proxy Celestia and Izzy representing the Moon (MOONbow) and by proxy Luna,
Argyle being mainly blue, while Sunny has no blue in her design, and while Izzy has blue hair
Argyle teaching Sunny other-pony-kinds-friendly values, as if he knows the stereotypes others believe in aren’t true, because he’s mingled with one hehe
The unicorn in Sunny’s letter being identical Izzy color-wise, as if someone who’s seen Izzy before had influence over Sunny while she drew the picture
And obviously neither Sunny’s mom nor either of Izzy’s parents being established in the movie
So what happened in the hc:
Argyle, as a historian and truth seeker, goes out and meets Elsie, also exploring, under the tree
They get to know each other and start meeting regularly
Bingo-bango
Elsie shows up with belly, they decide if the foal is an earth pony they will be raised by Argyle in Maretime Bay and a unicorn will be with Elsie in Bridlewood
Elsie shows up with two foals, Sunny and Izzy, they decide to go their seperate ways, one with each foal, to save them from the circumstances
Elsie dies shortly after, leaving Izzy to get raised by Bridlewood as a whole