Casual Japanese noun meaning Older Sister. Onee-san is the formal form of this noun.
Onee and Onii (Older Brother) do not have to have the ending -san, -kun, -chan, or -sama
See Nii-San; Onii-san; Ototo-kun; Imoto-chan
Nee-san, when are we going to the mall?
Onee-san, watashitachi gaitsu mo^ru machi ni iki masu?