i had a blog post about the best Onee-sans in anime a while back:
http://animerefugee.ikimashou.net/?p=4
My Top 10 list in no particular order:
1.Chikaru-sama – Strawberry Panic 2.Aya Roznovsky – Battle Athletes
3.Asa Shigure - Shuffle
4.Mikage Tsumura – Jubei-chan 2
5.Millay Ashford – Code Geass
6.Melissa Mao – Full Metal Panic
7.Mahoro - Mahoromatic
8.Misato Katsuragi -Evangelion
9.Coco – Melody of Oblivion
10.Raquel Kasull –Scrapped Princess