little sister and older sister

Okay so I'm just a little confused. How do you:

address your older sister?
your younger sister?

and what do you call your older/younger sister when talking about them, not to them?

and preferably in slang or in terms that are more common for teens to use

thank you!! :)

Addressing (talking to them):
your own older sister: o-nee-san, nee-san; aneki (slangish, used by guys)
your own younger sister: (You are likely to use her name)

Refer to your own older sister: ane
Refer to your own younger sister: imouto

Referring to someone else's:
older sister: o-nee-san, o-nee-sama
younger sister: imouto-san

when is 'onee chan' used then? I've heard it being used when addressing your older sister? usually translated as "sis"

"O-nee-chan" is more informal than "o-nee-san" that I wrote above :) So, yes, "sis" (to your own elder sister) in a friendly tone.

thanks for all your help!