What on Earth even are these words?!

Nii-chan. Okaa-san. Onee-san. If you've watched anime, you've heard these words, and depending on the subtitler, you might've seen them, too. But what do they mean? Well, one might say that 兄 (nii) or お兄 (onii) means "older brother" - and literally, it does. However, it gets awkward when a character goes up to their neighbor and says:

こんにちは、ジョン・ドー兄さん！
Hello, Big Brother John Doe!

...

That sounds... unnatural, to say the least.

Onii and Onee

Let's cover the basics - we have お兄 (onii) for older brother and お姉 (onee) for older sister. While you can attach keishō to these, the「お」(o) itself is already an honorific. So you can actually drop it and just use 兄 (nii) and 姉 (nee) instead.

兄さん！
Nii-san!

However, there's more to it than just that. Onii can be used to refer to boys you're close with and onee can be used to refer to girls you're close with. In fact, sometimes in anime, TV shows, and movies, these will even been used to refer to people they don't even know. (See Godzilla Minus One or the first episode of My Hero Academia.) In addition, they can also be used as keishō (did you read about that?) and attached to people's names - just like in the first example.

こんにちは、ジョン・ドー兄さん！
Hello, John Doe-niisan!

It's important to note that often when attached to someone's name, the "o" honorific is dropped.

Ani and Ane

兄 (ani) is another word for older brother, and 姉 is another word for older sister. Typically, these words don't have honorifics attached to them. They aren't as widely used as お兄 and お姉, however, they sometimes are. You might also hear 兄貴 (ani-ki) or 姉貴 (ane-ki) - these are more rough versions of ani and ane.

おい、兄貴！
Hey, ani-ki!

Imōto and Otōto

These two words are the exact opposite of onii and onee - 妹 (imōto) is younger sister despite being the one that has the "i" sound, 弟 is younger brother, the "o" honorific is not used in any circumstance, and imōto and otōto are generally not used to refer to people except your actual younger sisters and brothers. And they also aren't usually used as honorifics either.

やめろ、弟さん！
Stop it, otōto-san!

Okaa and Otō

These are the 2 words you don't want to remove the "o" from - why not? Because I am here, and... no. Just kidding. お母 (okaa) means mom and お父 (otō) means dad. Here's an example sentence that sounds like something a little kid would say:

お母さん、お父さん！見てくれぇ！
Okaa-san, otō-san! Watch this!

Obaa and Ojii

お婆 (obaa) means grandma and お爺 (ojii) means grandpa. Plain and simple.

クリスマスクーキーくれてありがとう、お婆さん！
Thanks for the Christmas cookies, obaa-san!

But be careful - this can commonly be confused with oba and oji, which are NOT the same!

Oba and Oji

Not to be confused with obaa and ojii, おば (oba) means aunt, and おじ (oji) means uncle. Sometimes, they can also be used (informally) to refer to older people, similar to how onii and onee are used. Honestly, for a while, I thought that obaa and ojii were used for that - proves how easy they are to get mixed up!

誕生日プレゼントどうも、おばちゃん！
Thanks for the birthday presents, oba-chan!