We can use elder and eldest as an alternative to older and oldest to explain the order of birth of family members.
We use elder when we speak about two brothers or sisters (also called siblings), or two sons or daughters:
- his elder brother, his elder sister
- their eldest son, their eldest daughter, their eldest child
- the elder of the two daughters
- the elder of the two
- the elder of the two siblings.
We use eldest when we speak about three or more brothers, sisters or siblings:
- the eldest of three children
- the eldest son
- the eldest of the four siblings.
- Diana Spencer-Churchill was the eldest child of Winston and Clementine Churchill.
- Mike McCartney is the elder brother of Paul McCartney.
- I’m the eldest of three children.
- My eldest brother is ten years older
Compare elder and older, eldest and eldest:
- He’s their eldest son.
He’s the oldest kid in the class.
- Jo’s my elder sister.
She’s three years older than me.