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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?
learner
Posted 2014-11-08T11:06:05.370
Reputation: 6 070
That's what I believe, but because elder/eldest have a special use and special grammar, and because of Swan's note I am having some doubts, and needed a conformation preferably by citing references or clarification from knowledgeable natives speakers like you. – learner – 2014-11-08T11:25:23.337
You know what I think I am mistaken about my comment and still confused. Just let me gather up my thoughts and comment again! – learner – 2014-11-08T11:27:21.333
How many thoughts do you have? Two? or Three? :-) – Tᴚoɯɐuo – 2014-11-08T11:28:21.250
They do NOT have special use and grammar. It's a matter of comparative/superlative. – Tᴚoɯɐuo – 2014-11-08T11:29:16.693
Wouldn't you say:"let me pull my thoughts together"! I'll ask you about elder stuff in a moment ;) – learner – 2014-11-08T11:37:18.570
1Yes, that was idiomatic. I was joking about two/three apples|siblings. – Tᴚoɯɐuo – 2014-11-08T11:40:32.830
Thank you @TRomano. I get it now after a second reading of Swan's words. Anyway, I had to ask here to clear all the doubts. – learner – 2014-11-08T11:45:08.763