My aunt was only four when my parents were married. She cried when they wouldn't take her on their honeymoon. She was six when I was born and more of a big sister - at times a bossy big sister - than an aunt.
She helped me make some major transitions in my young life: she was at school when I started and helped me settle in; at Girls' Brigade when I was there (along with her middle older sister, my mother and my grandmother who were all leaders); at youth group when I started and would drive me around and advise me about boys (usually against them - I don't think anyone was good enough for her 'little sister'). She taught me to play the piano and it is a love we have shared to this day. I was bridesmaid at her wedding, and several years later we were both expecting at the same time (her youngest son is only five months older than my eldest).
I didn't have to call her 'aunty' like I did my other aunties because she was more of a big sister than an aunty. How do you define a relationship like that? To say 'my aunty passed away' suggests a more distant relationship than we had (and I have already lost another aunt this year and know that such a relationship is not 'distant' at all); but to say 'my big sister died' is less than the truth too.
Perhaps I'll just call her 'my big-sister aunty'.