How do I unlock Rosalina in Mario Kart Wii?

According to this guide ,

Rosalina: Get a “One Star” rank or better in all Mirror Mode Wii-Cups. Or if you have a saved game file from Super Mario Galaxy, you should win 50 Grand Prix races.

You can also get Rosalina by finishing a total of 4,960 races.

There are two ways: have a Super Mario Galaxy save file and she will be unlocked after 50 races or get 1 star in all 8 Mirror Mode Cups.

Yes, according to this site: http://www.mariowiki.com/Mario_Kart_Wii

Rosalina

Have a Super Mario Galaxy save file and play 50 races, attain at least 1 Star Rank for all 150cc Mirror Grand Prix cups, or play 4,950 races