"What grade is your older sister in?"
あなたの means "your" or "yours," あなた means you. It's normally omitted because when you're talking with another person or group of people it's normally obvious who you're talking about. Because it's uncommon, it also sounds rude, or even accusing if you use it in every sentence. Try to use it only when you feel that it makes the sentence clearer.
I think it was in the tips a few lessons back. For a whole run-down from a native speaker with a lot of suggestions about where you might want to use it, you can try Maggie-sensei: http://maggiesensei.com/2011/09/05/is-%E3%81%82%E3%81%AA%E3%81%9Fanata-ok-to-use-2nd-person-pronouns-nicknames/
あなたの is "your." Anata no.
あなた is "you" and the の "no" particle marks it as possessive.
Because お姉さん (おねさん: onesan) is used to talk to somebody else about their own older sister, you can omit あなたの since it is redundant to say your your older sister.
Note: Based on my understanding of the tips section, you refer to somebody elses older sister as お姉さん, but you also call your own older sister お姉さん when speaking to her directly or to your family about her. BUT if you are telling somebody else about your older sister, you use 姉 (あね: ane). I believe it is to humble yourself and not imply that your sister is of a higher social status than the person you are speaking to.
Somebody please correct me if I am wrong.
i wrote お姉さんは何年生ですか and it wasn't accepted
EDIT: wait, i have wrote あなたのあ姉さんは何年生ですか and it wasnt accepted either... what...?
EDIT2: ok i finally got past this, i have now realised i mispelled お姉さん before, but i don't understand why it wasnt accepted the first time without the あなたの. (i copy and pasted the answers here, so the ones you see here are exactly the ones i wrote before)
It is better to use おねえさん (お姉さん, oneesan) to refer other person's elder sister. This expression is always respectful. Without "o", namely "neesan" is a casual expression and is only used in a limited situation.
When one refers to his/her own (elder) sister, the term "あね" (姉, ane) is used as a humble expression. Do not use おねえさん (oneesan) for your own, except addressing your sister directly.
e.g. My elder sister is a university student
Correct: 私の姉は大学生です (Watashi no ane wa daigakusei desu)
Odd: 私のお姉さんは大学生です (Watashi no oneesan wa daigakusei desu)
Sorry dude, but if it is odd, many sources are using it incorrectly. Many people refer to their own sister as お姉さん outside of formal circumstances. As you mentioned, addressing your big sister directly you would call her お姉さん (or 姉上 for people overly archaic).
Many people I know (Native speakers) would use お姉さん rather than 姉 in regular conversation. But this is only anecdotal evidence.
From Goo辞書: 1.（御姉さん）自分の姉や他人の姉、あるいは、家庭内で姉にあたる人を敬い親しんでいう語。 (Onee-san) One's own older sister, another person's older sister, or a person within one's household who functions in that role.
From Weblio 英語例文 あなたは僕のお姉さんみたいですね。You are like my older sister.
According to 三省堂ウェブディクショナリー (Sanseido Web Dictionary) it is simply 姉の敬称 The polite way of saying あね.
There are books titled ぼくのお姉さん and 僕の姉ちゃん. Are these authors and publishers odd?
I will agree that 姉（あね）is more formal and proper, but that does not make お姉さん wrong or odd.
It is quite important to use humble/polite expressions distinctively and properly. It is one of unique (and tough) features of Japanese. Even if お姉さん is a polite expression, we do not use the term to mention one's own elder sister (in general).
The general rule is, use 姉 as a humble expression to mention one's own elder sister, particularly in a formal expression, as I already stated above.
You cited an example "あなたは僕のお姉さんみたいですね". However, this does not actually mention one's own elder sister, but another person than your own elder sister. Thus it could be お姉さん and does not sound odd.
It is sure that お姉さん is polite way of saying あね, but one should not use it when he/she mentions his/her own elder sister, when talking with other people outside his/her family (particularly in a formal conversation).
The book titles "ぼくのお姉さん" or "僕の姉ちゃん" would be accepted on a friendly note, but never common.
To be on the safe side, use always a humble expression to denote one's own relative, e.g. 母 instead of お母さん, 兄 instead of お兄さん, 叔父 instead of 叔父さん, 祖母 instead of おばあさん.
日本語では自分の近親者を呼ぶ場合、原則として謙譲語を使います。elder sister に対応する丁寧語は「お姉さん」ですが、謙譲語は「姉」です。近親者に対し「お姉さん」の語が許される例外は
I fear that we are overly confusing the original poster of this question and straying farther from the question which was asked.
I do concede that 姉（あね）is considered the proper way to speak about one's own sister to a third party. If given the choice of whether to use あね or おねえさん in this case, I would agree that あね is correct.
I would, however, maintain that not everybody speaks properly in everyday conversation. Just as, 'I'm good' instead of 'I'm well' as a response to 'How are you' is lamented as the loss of the ability to properly use the English language, there are similar examples in Japanese. 正しくないのかもしれませんが、私にとっては「変」だと思わない。正解の言葉扱いと日常扱いの違いじゃないですか。
I really enjoy and appreciate this kind of discussion and do not want you to think it is adversarial. 感謝です。仲良く行きましょうね、Okamotoさん。
Is it okay for you to refer to another person's older dister as just neesan? Not by adding the honourific "o" in front?
No, you should use the honorific 'o'.
I would like to appreciate your comment to make this community better. I quite agree that languages are always changing, as you pointed out.
However, the distinction between 姉 (humble) and お姉さん (honorific) is still effective and dominant. It is not a matter between "traditional, classic" and "colloquial, daily". This distinction is taught in elementary school (regrettably, not everyone can use two expressions properly).
I would be glad if this discussion helps you or other learners. Good learning!
Yes I agree with Okamoto-san that for beginners better stick with 私の姉 and xxさんのお姉さん to be "safe" from being impolite.
For advanced usage -
私のお姉さん is OK if it is in one's own diary for example.
xxの姉 is appropriate if xx is your colleague and you are talking to an external client for example.
The most common usage of 'big sister, namely おねえさん（お姉さん）, already includes the polite prefix お. The simpler form is ねえさん, but this form is not conventionally used and can drastically change the nuance.
御 is read as either お or ご depending on the word it is used with. In this case, ご would be not only incorrect but superfluous (akin to calling Sir Mixalot 'Sir Sir mixalot').
I omitted あなたの in my answer (I put simply おねえさんは何年生ですか？) and it was marked as incorrect. I thought that this might be a case in which the context would make it reasonably clear that the reference would be to the elder sister of the person being spoken to, so saying "your" would be unnecessary. Am I mistaken in that supposition? Is there another reasonable interpretation that might make it unclear what was intended? Thanks.