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