a sister/brother/son/daughter who is older than the other sister(s), brother(s), etc.

Her parents and elder sister stayed behind in Poland when she came to America.

He always looked up to his elder brothers.

He had to share a room with his elder brother.

As the elder son, everyone expected him to take over the family firm.

He always had an uneasy relationship with his elder daughter.

See also: elder adjective