eldest 和 oldest 有什麼關係?

Elder and eldest can be used instead of older and oldest to talk about the order of birth of the members of a family. They are only used attributively (before nouns). Compare:

My elder/older brother has just got married.
He's three years older than me.

His eldest/oldest daughter is a medical student.
She's the oldest student in her year.

Elder brother/sister are used when a person has only one brother/sister who is older, eldest is used when there are more. An elder son/daughter is the older of two: an eldest son/daughter is the oldest of two or more.