This is Princess Dusk. I’m unsure if she should be Dawn’s little sister or a stand-alone character from another universe entirely.
 
Dusk is a very serious, very introverted mare. Despite her upbringing with her two mothers, Twilight and Celestia, Dusk does and still has difficulty making friends and recognizing the magic in friendship altogether. Dusk prefers to be alone, in a library, or with her mothers. She often goes without her crown and horseshoes in order to stave off attention, even though it hardly works since everypony knows she’s the daughter of the princesses.
 
Dusk has the ability of sunset magic, giving a push to the sun to go down and the energy to the moon to go up, which at those times her magic is boosted. (In a nutshell, she could stave off the morning for an hour or so.) Luna is Dusk’s favorite aunt and loves staying up all night with her to watch the glorious night.