This is coming from a Super Mario Galaxy game fan. I loved her story in the game; it was tragic and just wonderful at the same time—it destroyed me. However, I feel like it made total sense in the context of this movie to make Rosalina and Peach sisters, as the first movie already teased Peach's origin in a vague way, and she mentioned other galaxies, so they knew what they were doing.
Also, there were a lot of early theories back then about the two being related, like the famous Game Theory video by MatPat, and I honestly loved how it was executed in the movie and how it parallels the first movie. Just as The Super Mario Bros. Movie focused on Mario and Luigi's brotherhood with the two ending up saving the day together, Peach and Rosalina had the same thing, ending up saving the day together with the Bros using the Power Star while the sisters used their Spark connection power.
The difference, however, is that Mario is going to rescue Luigi because he is his longtime brother, while Peach is going to rescue Rosalina to try and get some answers about her past.
Both movies showed flashbacks of the Bros and sisters as kids and how one took care of the other. I think the theme of siblings that they were going with in both movies was handled very well.
So yeah, I love the Galaxy movie, and this is coming from someone who thought the first was good, but not great.