Theory: Rosalina is Mushroom Kingdom Royalty

Abstract
Rosalina is a past princess of the Mushroom Kingdom, with her brother being a direct ancestor of Princess Peach.

Arguments

 * Argument 1: Rosalina was a princess as a child. This is indicated by the crown she is seen wearing both as a baby and as a little girl in Rosalina’s Story.


 * Argument 2: In the final chapter of Rosalina’s Story, “Final Chapter: Family”, Rosalina decides to “return home once every one hundred years to nap in my favorite sleeping nook”, and it goes on to say “the comet visits the girl's home planet once every hundred years, its proud white tail glittering in the sky.” These statements heavily imply that this is what caused the Star Festival, an event which affects the Mushroom Kingdom specifically, indicating that the Mushroom Kingdom was her home.


 * Argument 3: Alongside the aforementioned text, there is a picture of the comet mid-flight which looks near-identical to the comet’s appearance as it flies over the Mushroom Kingdom during the intro of Super Mario Galaxy, further proving the previous point.


 * Argument 4: Rosalina later supports this idea when Mario enters the Gate for the second time, stating “This planet… it’s very dear to me. I looked forward to visiting this planet with the Lumas every one hundred years…”


 * Argument 5: In “Chapter 7: The Telescope”, Rosalina uses her telescope to see her home planet, before recalling memories from back home. Both the initial vision of her home that she sees through the telescope as well as every subsequent memory shot have a silhouette of a castle that matches the iconic shape of Mushroom Castle exactly, with a tall central tower and two smaller side towers visible from the front.


 * Argument 6: Rosalina’s Mother, as she is seen in “Chapter 4: The Dream”, looks very similar to both Rosalina and Princess Peach, perhaps implying a relation between the two. This was intentional, with Yoshiaki Koizumi stating in an interview with PRIMA that "We contemplated the idea that Rosalina was related to Princess Peach, so that is why their features are very similar. Her long bangs represent her outward strength and inner sorrow and loneliness."


 * Argument 7: Rosalina’s brother is mentioned briefly in “Chapter 7: The Telescope”. Since he is her only other known relative aside from her parents, it is most likely that he continued the royal line, eventually leading to Princess Peach’s reign.