Translation:I'm not an only child; I have an older brother.
Elder or older? My understanding has always been that 'elder' is used when describing the relationship between two siblings of the same sex. Eg: my younger son has an elder brother, but my daughter has two older brothers. So in the context of this this translation I think older or elder should be acceptable.
- French phrase says nothing about daughters. So "child" is wrong.
- "elder" is the most standard word in this situation, MUST be accepted.
- Only child is "THE only child". Why? Because he/she is ONLY, ONE, SOLUM FILIUM. Three issues for one short and simple phrase is way too much. In year 2000 people used "the only son" almost 3 times more frequently than "an only son" according to Google Books.