"My older brother is inside."
Why is ”あにはなかにいます。” wrong? I'm pretty sure that also conveys that "my older brother is inside."
Acceptable to whom? If your answer is Duolingo, the answer depends on whether or not their system currently allows it.
「中にあにがいます。」is a generally accepted equivalent to 「あには中にいます。」. The latter actually sounds more natural.
We learned at school that the います/あります structure uses が, while は is used with e.g. です
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?
It is a preposition. 'Inside the house' is implied, as in 「家の中にあにがいます。」. This construction suggests that the question is not 'Where's your brother', but 'Is anyone inside?'
The more natural way to say this would be 「あには（家の）中にいます。」.
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.
います/あります stand for "there is" and is a certain scentence structure that is often unsed instead of the "です" structure. います is used for living thinks while the other is used for objects
Because of earlier lessons, everytime I finish translating this, my brain goes: "He's inside. . . He's inside what? Me? 'It'? No that doesn't sounds right. Why is he inside anything? That doesn't make sense! Oh wait, the inside..."
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.
に says where he is. の is a possessive. So that would mean he belongs to the inside which wouldn't make sense.
Just wrong. 'Ani ni' would be 'to my brother' or 'In my brother'. 'Naka ga imasu' would mean 'There is an inside/center', but imasu doesn't work for inanimate objects. 'My brother has an inside' is quite different in meaning.
How come this sentence reads "Inside my brother exists" and not "my brother exists inside"? Is there any way to tell how to start the sentence off? I've tried putting ani first before and was wrong.
Without knowing what you wrote, it is hard to tell you what was wrong. 兄が中にいます。should be fine, though.
I understand this frustration. Was Duolingo marking answers as wrong when they should be correct? Or was the sound doing strange things?
Of all the courses here, the Japanese course is the one that has often left me so frustrated that I swear at the computer and leave, so I understand the rant. Sometimes you just gotta let it out.