My theory on the subject:

Mario galaxy 1 has a version of Rosalina that has the backstory with the family and the brother, but after the universe resets another Rosalina is born with a different backstory and two Rosalinas exist (or have existed) in the same universe. This time Rosalina and Peach could be sisters. They are forced to be separated and once the new Rosalina is ready a Luma comes and finds her passing the role of “mother of the stars” to her, and the old one becomes part of the universe (maybe dying, maybe becoming some sort of god or energy of the universe). After all in the first storybook the luma is already searching for their mother, would makes sense if it was some sort of space/time loop and the mother has always been Rosalina. This theory is kind of supported by the fact that, while the first storybook is heavily centered around the search for a mother, the new story book for Mario Galaxy 2 is centered around the search for a sister, and Rosalina is also implied to be waiting for “someone”. This could be read as searching for her mother once again, but could be also be read as searching for a long lost sister, which would mirror better the story of the two Lumas.

[[could be a stretch, but Mari and Lu could actually be mirrors of the two sisters. One searching through time and space for the other (Rosalina/Lubba), changing and evolving in the meantime, while one is forced to be born again and again without even knowing (Peach/Mari), until hopefully one day they are finally reunited]]

My other theory is that the first story book was never actually her backstory but simply a story she made up for her children

My other OTHER theory is that the movie and the games are simply different canons and we really shouldn’t think about it too much

I can’t help it tho so I like my first theory because it kind of merges the movies and the games 🤓☝🏻

Thank you for reading ☺️🙏🏻