By placing ご (go) or お (o) in front of a noun, you can make it more formal (Go in the case that word is using "On" reading and o in the case of "Kun" reading). So in the case of cousin, いとこ (itoko) is Kun reading so it becomes おいとこ (oitoko).
Onii-san is the honorific while the actual word is ani.
사마
The honorific 'chan' is written asちゃん in Japanese.
'(Watashi no) itoko' is "my cousin" in Japanese.
No, I believe it's just Sensei.
itokoSay It-oko.
Shigeru Kajiwara has written: 'Suberanai keigo' -- subject(s): Japanese language, Honorific
Yasuto Kikuchi has written: 'Keigo' -- subject(s): Japanese language, Honorific, Usage
Generally speaking to be safe, you address them with the honorific "-san" if you know their name (if you don't know them very well, just use their last name with the honorific). If you don't know their name, there are other terms you can use.
Akemi Yamashita has written: 'Kaigai no Nihongo no atarashii gengo chitsujo' -- subject(s): History, Honorific, Japanese, Japanese language, Social aspects, Social aspects of Japanese language
Itoko
'Itoko.'