I need the words for:

Big Brother - Oniisan 

For the familiar effect which usually
occurs among family members, change the -san
to -chan. i.e. - Oniichan.

Big Sister - Oneesan 

As with Oniisan, you can (and normally 
should) change it to Oneechan.

Little Sister
Little Brother

In Japan, elder siblings usually call their younger brothers
or sisters by a shortening of their names into a "cutesy"
pet name appended to -chan. However, if you want to
get to it, "otouto" is for little brother and "imouto" is for
little sister.

Thanks.

Arigato gozaimashita.