kun doesn't really mean little brother. But it is usually used to refer to younger boys or by girls to refer to boys their age or of the same status. Sometimes, kun can also be used to refer to girls who seem to command more respect or girls who are more boyish. It is a casual way to call a guy. (From my understanding)
For sister, the basic/standard is onee-san. chan is a cuter way to refer to girls or a cuter way to refer to people whom they want to feel close to. (E.g. Shinichi's mother calls him Shin-chan instead of kun). If you watch shows in the olden era, you might hear younger people calling their sisters Ane or Ani(brother) which seems similar to Korean. They add a -ue behind instead of -san for a more formal address.
For older brother: Onii-san. -chan is another way to add a post-fix. The O- prefix is also a way to suggest respect. Many nouns and greetings of japanese start with an O-. Let's say, Oyasumi, Ohayou, Otousan, Okaasan, Otearai, Okaeri, Okane...
Father: Otousan
Mother: Okaasan
Aunt: Obasan
Uncle: Ojisan
Granny: Obaasan
Gramps: Ojiisan
Younger sis: Imoto
Younger Brother: Otouto
Something like that. Please correct me if wrong.