i was playing Katahane last night, in it Light and Wakaba refer to their mother as Kaa-san while Light refers to Wakaba (his older sister) as Nee-san and when meeting with Rien he says for them to call him Jii-san by commenting
If you did not know my name or who i was i would just be an old man to you correct?
^ (he's not saying that in a bad way mind you)
also in Fate/Stay Night, Sakura refers to Rin as Nee-san and the 2 of them are sister however Sakura was adopted out to prevent a potential fight over Magus Succession between them (like what happens with Touko and Aoko)
from my understanding, Okaa-san is Mother, Onee-San is Big Sister and Ojii-san is Grandfather, now while Kaa-san and Nee-san i can still determine refer to Mother and Sister i'm assuming Jii-san is something close to Grandfather as Rein in Katahane is almost like a grandfather to Cero and since Wakaba is close to Cero (potential love interest in one route) i'm assuming he doesn't mind being though of as a grandfather but that's seculation,
so i am wondering, what is the difference between Okaa-san/Onee-san/Ojii-san and Kaa-san/Nee-san/Jii-san apart from the obvious drop of O from each word