Luna stared at Stella. It wasn't that she didn't remember her. How could she forget? Stella had always been sour about being in the middle. She was passive agressive before she even knew the words passive agressive, according to Celestia.
Stella was almost the same age as Luna, and Celestia was much older than both of them. Stella had always worshipped Celestia like a goddess, but Celestia despised Stella to the point of trying to get rid of her multiple times. Stella had been crushed when she realized what Celestia was doing. Of course, it wasn't like Luna was just left out during all of this. Celestia didn't like it when Stella was born. Up until then, she had had all the attention and had accepted that she wasn't getting a sibling, to her relief. So when her parents told her, she kicked and screamed and wailed and whined and flailed and moaned and groaned. And so Celestia held a grudge against Stella from the beginning. Not big enough for anything monumental to happen, but it would blossom into a full-on rivalry. When Luna was born, a couple months later, it just made everything worse.
Celestia still didn't want yet another sister, but she had gone through it before and could handle it again. Whereas Stella could not. She kicked and screamed and wailed and whined and flailed and moaned and groaned, but nothing changed. Luna was born, and Stella hated her from that moment on. Stella was in fact so mean to Luna that Celestia want against her own ways and started feeling bad for Luna. Celestia lost it when Stella told Luna that nobody needed the moon, if they did they wouldn't sleep through it (this was when Luna got her cutie mark), which, if you think about, was a pretty hypocritical thing to say. Ponies slept through stars, too. Celestia decided she wasn't talking to Stella anymore. And she didn't. For a moon. Then they started talking again. But the point was that Celestia could perservere and do what she wanted.
When they had left for Equestria, they really hadn't meant to leave without Stella. She didn't tell anyone she was going to pee. But they were halfway there when Celestia remarked on how pleasantly the journey was going. Then they realized they had left Stella. They didn't want to go back for two reasons: Stella was a jerk and they were halfway there. The morning was foggy and the alicorn world was unplottable for non-alicorns. Though they did have wings and horns (they were flying to Equestria) , a damp morning like this would make it difficult, if not impossible, to relocate. They might end up flying crcles forever, and with all the dangerous villians walking around in those days, two young but juicy alicorns would be gobbled up like candy. They flew on.