'おばちゃん(Oba-chan)' means 'aunt/aunty' but also means "middle aged lady" in Japanese. For that reason, my sister refused to be called 'oba-chan' by my son Ichi (even though her children call our brother 'oji-chan' which means 'uncle' but could also mean 'middle aged man' in Japanese.) So, she introduced herself to Ichi as 'アウンティ(aunty)' not 'oba-chan'.
My nephews call me 'ねえねえ(nee-nee)' which means 'sister' in this case. It's because my first nephew was born when I was still 18 years old. Being called 'oba-chan' at the time did not feel very natural to me and my family. Even now, after I turned 30, my sweet nephews still call me 'nee nee'. But now, my brother and her wife are expecting a baby at the end of this year. I guess it's not very appropriate to call an aunt who is more than 30 years older than you 'nee nee (sister)'. The time has come - I'll be 'oba-chan' soon.
..... Or, maybe I can say I'm 'aunty' because I live in America? ;)