Another bit of grammar for you and oooh, this one’s complex. We love a bit of complexity, don’t we?
While older (and oldest – the rules for that are the same. Phew) has replaced elder in almost every circumstance, our language wouldn’t be English without an exception.
When talking about siblings, use elder. “I have one elder brother”; “He has two elder sisters” etc,
It gets confusing when you have other relations AND siblings. For example, if I have two cousins (who are siblings of each other), I might say “I have two older cousins [older than me]; David is the elder [of the two].”
Good, because we’re not done. You can’t just always use elder when talking about siblings. Sometimes older is still correct. For example:
“This is Jeffrey, he’s my elder brother [older than me]. I have an older brother [older than Jeffrey], Mike, but he’s not here today.”
“My elder sister [older than me, always was, always will be] is older now [older than she was before], but back in 1981, she was seventeen.”
I know. It’s confusing. If in doubt, use older – even if it’s wrong, the mistake is less likely to be noticed than misusing elder. Or study the extract from Fowler’s Dictionary below, and think yourself lucky that at least younger doesn’t have the same issue.