I confess I find it difficult to understand why so many English learners have a problem with this very straightforward rule. It’s obviously not explained very well, if at all, by our dictionaries, etc. Sorry about that!
It’s not so much about grammar as about pronunciation. The indefinite article is “a”. But when you put that immediately in front of a word with a similarly open vowel sound, it’s really quite difficult to pronounce.
That, and that alone, is why we add an “n” to the indefinite article when that occurs. But please note that some words begin with a vowel but are pronounced as though they begin with a consonant, and vice versa, and in those cases the article is either “a” or “an” according to the pronunciation:
a apple, a elephant, a idiot, a office, a umbrella
an apple, an elephant, an idiot, an office, an umbrella
a unicorn / a eulogy an honour / an hour
A bit silly question but if there are only two brothers why "an elder brother?" I know I am wrong but it sounds to me as "I have one elder brother who is one of my other elder brothers". When we have two things we use THE OTHER, hence "the elder brother". "An elder brother" is like "another elder brother". It's confusing.
Anyway how about this:
There are two boys in this family who are native brothers to each other: one is the younger brother and the other one is the elder brother.
There are two boys in this family who are native brothers to each other: one is a younger brother and the other one is an elder brother.