In his book section 176, 3rd ed., Swan says:
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.
That was not clear enough; I don't see that here. Does that mean you can only use it if you have only one sister that is older than you (you don't have two or more sisters who are older than you ), or you can only use it when you talk about the youngest sister (out of two or more) that is older than you?