It was near the beginning of the blog, but here was the initial look at Radiant Sunlight, Sunset & Twilight’s fusion.
So there, your explanation is out! In simpler words, a combination of Sunset’s Empathy Element, the Mirror Spells, Harmony Magic, (and of course love), led to the conception of the child. Like Discord said, it just took a bit of a while for the magic to manifest and set into development.
The Mirror Magic to reflect their forms, the Harmony Magic to bring them together, the Empathy Element Magic to give her her own soul, or spirit, and their love to give her life. Real deep stuff.