I would like to add my own first hand experience as an answer to you. My father's eldest brother was older than him by 10 years. My Uncle's 1st son was 18 years older than I was & the 2nd son is 15 years older than me. I was raised to call both of these 1st cousins as "Uncle so & so" out of deference to the age difference. The remaining 2nd son & I had & still have a big brother/little sister relationship & stopped calling him 'Uncle' when I was about 13 years old & using only his & his wife's given names. When his wife & I are talking about him in the 3rd person she ALWAYS refers to him as 'Uncle so & so' & I have to tell you that neither one of us really notice the distinction. When I introduce him to people, he is introduced as 'my Uncle'. My children call him 'Uncle' & I had to explain in detail, our blood & lineage relationship because they really did think we were siblings. He is now 83 years old & I thank G-D everyday for giving me the best Uncle/Big Brother a girl could have ever wished for. Although his parent's were really my Uncle & Aunt, they stepped up to the grandparent plate for me & my siblings. Sometimes our emotional feelings about people are the truths of the relationship & not necessarily the true lineage.