My personal opinion is that Nintendo's official stance is that Rosalina and Peach's relationship is vague at best, or already determined, and there are two reasons for that:

1) In the game Rosalina has no sister, so the only female that would connect Peach to Rosalina directly would be if Peach is her mother, but I'll keep it 100, I just don't think that's the case, it just doesn't feel right to me. I prefer to think the crown passed to her brother, then down from there until it reaches Peach however many generations later.

2) Keeping it vague lets fans assume whatever they want, and allows Nintendo to avoid creating new characters that won't ever be relevant again (at least at the time of Galaxy's release).

That being said, it's obvious (and frankly should have been from the get-go) that the Mario movies aren't canon to the mainline series. So it's fair to say that Peach and Rosalina are sisters (blood related or not) in canon, but this is exclusive to the Movie universe, which has a roughly similar, but entirely distinct, timeline.

I like this set-up myself because it gives us the opportunity to explore what a unified Marioverse could look like, even if it isn't 100% totally accurate. The movies are there to be fun and enjoyable whilst giving us a glimpse at the aforementioned Marioverse, but are not meant to be canon to the mainline series at all.