[Renica]

On the topic of this whole Onee-chan/ Onee-san thing...
Wouldn't it be more appropriate to call an older girl or boy Ane-san(For girls)/Ani-san or aniki (For boys)?

And on the subject of -chan in general, I have read that it is simply a word to be used towards close female friends. -kun is used towards male friends or people who are younger than you. (E.G., in Fruits Basket, Hatori refers to Tohru as "Honda-kun".)

Really the most improper thing to call someone is their given name with no honorific. It is considered rude to do that without permission from the addressee.
On the other side of the coin, if you have permission or are very close to this person (Lovers, spouses, very close friends) it can be considered a compliment, or show of affection to call their name without an honorific.

I hope I got that right. I am very sorry if I did not.