"My older brother is inside."
I believe you're over-extrapolating what you learned. Used directly before いる/ある, が should normally be the particle used to indicate existence, not location. There are also considerations regarding what the topic and subject are.
Do you have an example of what (specifically) you think is wrong and how it should be fixed?
I know this question is old and I am just a fellow learner, but...
I think it helps to think of 中 【なか】 as a noun which takes the particle に. (Not sure if that is actually the case...) So if you know something like 家に 【うちに】which means "at home", you can think of 中に as "'in' inside". (= "at the location of 'inside'") This sounds strange, but I found it useful.
兄弟（きょうだい）, while technically 'brother(s)', also means siblings, as masculine is the default for non-gendered nouns. 姉妹（しまい）would be sister(s).
So, if you're just saying, 'I have 3 siblings', or asking 'Do you have any siblings', you can use 兄弟. If you want to be specific and ask about sisters you can use 姉妹. You can also use 男兄弟 (male sibling/s) or 女兄弟 (female siblings).
Main point: If you are referring to one individual with a definite relationship, (older brother, younger sister, etc.) you should use that specific word. You do not want to substitute a neutral term when the specific term is already known.
Very casually, you could say 「おにいさん（あに）は中です。」, but it is not recommended due to the improper grammar. に + です will not work together, as に indicates location as does います and they fit together as a set pair. です just indicates existence, not location.
To put it another way, going to the trouble to introduce the particle に shows an effort to form a more grammatically correct sentence structure, then using です when います is the correct form just makes it sound schizophrenic. Mixing up casual and proper forms will do that.
Particles are post-positional, they act as a suffix for the thing they describe.
に goes after the location/time/target of movement
が goes after new information, the subject that is doing the action/being
Your sentence would then say something like "In/at my older brother there is an inside" with "brother" as the location and "inside" as the thing that exists. (Though in that case the animate existence verb います would be incorrect since "inside/middle" isn't a living thing)
You can flip the place of the subject and the location but you also have to move the particles with them.
兄が中にいます or 中に兄がいます
兄が - Older brother (subject)
中に - Inside (location)
います - Exists (animate)
Because the sentence already ends with a masu-form verb （います）.
This explanation is a bit simplistic, but you use です when no other verb is present at the end of a sentence to function as 'is'.
コーヒーがあります There is coffee. コーヒーです - (It) is coffee
Later, you might learn how sentence-ending verb forms can change to include です, but that is a more advanced and nuanced usage.
You can't place a particle directly in front of です
です equates one thing with another
AはBです - "A=B" - A is the thing you are talking about, marked with は, B is the comment you are making about A, directly linked to the copula です.
に is a location particle, which tells where something exists in time/space or the target of an action.
Your sentence reads as incomplete because it introduces a new component C - the location. "Inside" is marked as the location where the comparison takes place, but you are missing your B, the thing that is directly being compared to "my brother" that would be marked with です. "Inside, my brother is....?"
CにAはBです "A=B in C"