This article will show you how to unlock Rosalina without Super Mario Galaxy. And it actually WORKS !!!
Steps

Step 1. Turn on the Grand Prix mode
You must play with one player when you do this

Step 2. Select the Mirror module
If you don't have one, you'll have to unlock Mirror Mode on your Mario Kart Wii.

Step 3. Choose your character and cards, no matter which one

Step 4. Be prepared to win every heat in every race
This may take a while since 150cc is the second hardest.

Step 5. Race in all Mirror races, then you will get a star rating in each race, then you will get Rosalina
Advice
- You must be first in all heats of all races.
- Be prepared to make more than one attempt.
- If you have a Super Mario Galaxy saved file, you only need to win the Grand Prix race streak without having to play the Mirror Cup.
- You can also get Rosalina by playing Super Mario Galaxy, then you will receive it as a gift from Super Mario Galaxy.