My best friend's parents when I was a teen wanted to be called uncle and aunt. I was there A LOT, they were a big part of my life and always good to me, so I always called them aunt/uncle because it made them happy. Felt pretty weird to me though because I didn't actually call my uncles and aunts by the title :-).... Was especially weird at my wedding, with my actual aunts and uncles there thinking "huh?"