Can the eldest sibling of three males call his younger brother "my older brother"

Had this conversation with a 26 year old male. He has finished a degree at Uni and told me I was wrong when I said you cannot call your younger brother your older brother. He is only older son to your mother who would refer to her three sons as "my youngest son, my older/elder son and my oldest/eldest son. He was so passionate about this (my three sons also told me I was wrong) I had to stop but at 47 I am wondering what is correct.

To take it from from a linguistic boffin's standpoint, I guess that "older" is a deictic word: its meaning varies depending on usage. There's a handy illustration at Wikipedia's page for deixis. When you say "my older brother" you put yourself in the deictic centre. Your listener then should figure out the position of your brother relative to that centre.

As WS2 rightfully pointed out, you may with a degree of certainty say "he's my older brother" to an acquaintance who knows that you're the oldest of the flock, but not to just any John Doe.

That's because deictic words require contextual information to be fully understood, and your acquaintance will know that while you're at the deictic centre there's no way for you brother to be at the.. er.. deictic "above", because there's a.. hm.. "deictic ceiling" of the fact of your being the oldest.

It would depend to whom he was talking. If it was someone who knew the family well presumably he would mention his brothers by name. If it was to someone who just happened to know he was the eldest, 'older brother' might just scrape in as acceptable speech. But if it was to someone who didn't know him or his family from Adam, for clarity's sake he would need to say 'The older of my two younger brothers'.

Here's one for you that ensued in a big argument. I have four older brothers. I referred to my next eldest, the third eldest as my youngest brother. I was told that I was dead wrong because he was not younger than me. But I am not wrong. It dosn't matter how you cut it he is still my youngest brother. I never said he was my younger brother, only the youngest of my brothers. Of my four brothers as a group, he is the youngest. Can anyone out there logically contest this?