elder and eldest

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. (not … elder 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.