Japanese - What is the equivalent of onee-chan for someone that's younger?

Question

I know onee-chan is older sister and onii-chan is older brother but I'd like to know the equivalent for younger siblings.

I've heared Onee-chan and nee-chan used when referring to friends who are considered like sisters (similarly for boys with nii-chan) But the friends always tend to be the same age or older, so what could I use if the friend is younger but still like a sibling?

Answer 1

Little Brother: otootosan
Little Sister:imootosan

Answer 2

There is a word:ototo-bun/imouto-bun. This is when you consider the younger person a quasi-sibling. Like you've grown up or apprenticed under the same mentor. You may not be blood related, but at heart, your siblings.

Answer 3

Older people and relatives get titles and other non-names. Younger children and non-special people get called by their names.

Answer 4

+kun
+chan