LO>Additionally (this is just my observation from when I was in Japan as an
LO>exchange student), it seems that 'imouto' is used more often than
LO>'otouto'... 

This must have something to do with male dominancy. I
think that calling someone by name is a greater show
of recognizing that person than by the formal term.

Don't take my word for it, though...

LO>Additionally, 'oneesan' and 'oniisan' are not always used alone... they
LO>can be tacked onto a name like -san or -chan or -kun.  AND, to further

To further add to that, 'oneesan' or 'oniisan' falls under a
class of words called honorifics such as sensei, kun or san.
Anyway, this seems to be a more formal way of calling a person.

LO>confuse matters, they can also be used not just for biological siblings,
LO>but for people you respect like an older sibling... :)

The cultural interweavings are limitless. :)

The same is true in my country where older people whom
you respect (or, more importantly, who take care of you) 
are also called "elder brother" or "elder sister."