So I've just found out that apprently お兄さん and お姉さん aren't used for your older
siblings, and only for other people's siblings or just other people older than
you. Is this true? This doesn't seem right but I would like to confirm if this
is right or wrong

Does that only mean 姉さん/兄さん, お兄ちゃん/お姉ちゃん etc... only work here? I guess what
I'm asking is it rare or strange to use お when talking to your own older
sibling? I believe that お just make words more formal so what about here?
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nuances
Is calling your older brother/sister お姉さん/お兄さん wrong?
It’s not wrong to call your elder sibling お姉さん or お兄さん, of course.

It’s just that you should use 姉 or 兄 to refer to them when you are talking to
someone outside of your family, or outside of whatever you consider to be your
“inner group”, in relatively formal settings. If you are talking to a friend,
it’s totally fine to call them お姉さん or お兄さん (though お姉ちゃん or お兄ちゃん is more
likely).
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The rules of “proper” Japanese is that one should address one's relatives
directly with “お兄さん”, “お母さん” and such, but use “兄さん" and “母さん” when speaking
to others about one's relatives, and use “お兄さん” and “お母さん” to refer to their
relatives. This is because in theory using the “お” elevates the status, and
using that on one's own relatives when speaking to outsiders is supposedly
rude, but one is not speaking to an outsider when speaking directly to one's
own relatives, where one can elevate them again.

In practice, Japanese people completely ignore this and use “お兄さん” all the
time to refer to their own relatives when speaking to others.

Another “rule” is that “父” and “母” can only be used for one's own parents, and
“お父さん" and “お母さん” must be used for the parents of others; but again, Japanese
people do not follow these rules and they're quite happy to use these terms
for the parents of others. In the strictest etiquette even, one cannot use
“お父さん” or “父さん” ever on one's own parents and must use “父”, which is again
ignored. Even stricter is that not even “(お)父様” cannot be used for one's
parents and “父上" must be used, and as rare as “(お)父様" is outside of fiction,
“父上" is even rarer in actual usage.
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What you said is completely true for 弟さん and 妹さん. These are always (lightly)
respectful ways of referring to **someone else's** younger brother/sister. You
can never use them to directly address **your own** younger brother/sister,
regardless of what type of person you are.

However, **it _is_ possible** to use お兄さん and お姉さん to address **your own**
older brother/sister, although it's not very common. You need to understand
how each phrase sounds and who the typical users of each pronoun are.

When used to directly address **your own** older brother/sister (imagine
you're alone at home with your brother/sister and no one else is listening):

  * お兄ちゃん and お姉ちゃん are the most common, and you'll hear them most of the time in real life, at least in urban areas. My sister used to call me using this when she was little.
  * 兄ちゃん and 姉ちゃん are the next common, and sound relatively tomboyish and/or rough. Male speakers tend to prefer 兄貴 and 姉貴.
  * 兄さん and 姉さん sound relatively mature. Luigi always uses 兄さん when addressing Mario.
  * お兄さん and お姉さん sound fairly polite, and you won't hear them often outside of fiction. The main users are stereotypical [ojousama](https://www.japanesewithanime.com/2018/05/ojousama.html), courteous gentlemen, elegant old lady and so on in fiction.
  * お兄様 and お姉様 are even more polite. They sound natural only when a royal princess or a daughter of a conglomerate is talking to her own older brother in fiction. 兄様 and 姉様 are almost the same.
  * You might hear [super rare ones in fiction](https://en.wikipedia.org/wiki/Sister_Princess#Twelve_sisters).

Note that many people choose to use nicknames instead of these words. I
haven't explained cases like
[this](https://japanese.stackexchange.com/q/17886/5010). When referring to
your brother/sister in formal conversations with outside people, you should
use 兄/姉.
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