Translation:My older brother
兄 (あに) referres to the only the older brother of the speaker (so only your brother). when saying 兄, everyone knows, you are talking about your own older brother. When talking of someone's elses older brother or an unknown young man in general, 兄さん (にさん) is the word you are looking for. お兄さん (おにさん) is an even politer version of 兄さん due to the honorific お which makes quite a few forms of adress more polite. Hope that helps
For middle-aged men, it's おじさん (lit. "uncle"), and elderly men get おじいさん (lit. "grandfather").
Young women have おねえさん. For middle-aged women, there's おばさん (lit. "aunt"), and elderly women have おばあさん (lit. "grandmother"). You might want to err on side of caution with the vowel sound on this one in particular ;)
Interestingly, I think when you are using these "titles" with strangers, relative age isn't really the deciding factor, absolute age matters more; an elderly man would still call a young man おにいさん.
There is also another alternative when you can't remember someone's name: it's somewhat common to address people by their professions. Of course, this is more acceptable for some professions over others. For example, it's extremely common (even if you remember their name) to call a teacher or a doctor 先生, or to call your sushi chef 板前さん (いたまえさん), but it's much less common, for example, to call a fisherman 漁師さん (りょうしさん) or a mechanic 整備士さん (せいびしさん).
I think there are alternatives but I don't know them yet. I started studying japanese not long ago, so maybe someone else can help? But I believe that you should be able to express yourself as polite as possible, as you can offend others easily if you don't. Remembering ones name is also a token of politeness as talking of someone you already know with words which are used to adress those you don't know, signals the other that you don't really much care about such things. Naturally, everyone cares about such things differently. So far my thoughts
-You wouldn't say ani-san, just ani (兄) = your own older brother. -Whereas onii-san (お兄さん） = anyone's older brother -You would also address your own older brother as お兄さん (as in "hello, お兄さん") -The お is polite and can be dropped to just 兄さん to be less formal
Note: お兄さん can also be any male who is old enough to be your older brother, usually one you know somewhat well. e.g. "the お兄さん who works at the restaurant" or "the お兄さん is my neighbour"
Yes, but keep in mind:
弟 (younger brother) in Japanese is pronounced おとうと, so the middle "o" sound is long.
君 (くん) is a familiar name suffix, meaning おとうとくん would only be used with children (if you are significantly older than the listener and the person you are referring to) or the listener is someone you are close to (in age and/or status) and you have previously met their younger brother.
You generally wouldn't address your (or someone else's) younger brother directly like this. When addressing your own younger brother, it seems distant; being older, you don't need to be so formal. When addressing someone else's younger brother, if they are younger than you, then it's the same problem as with your own younger brother, and if they are older than you, you should be using more polite terms (usually their name + さん, or お兄さん).
In the Japanese sentence, there is no word for "he". More importantly there is also no verb in the Japanese sentence. If there was a verb, we could infer the topic ("he") based on the context, but without the verb, this is a phrase or sentence fragment so we can't possibly translate it into a full sentence.
There used to be to many "vocabulary" answers like this. Actually, these sorts of questions should be given to show all the uses of a particle before making grammatically rich sentences with new vocabulary. It would help with vocabulary while being less overwhelming, too.