I say you are depending on the relationship. I have a girlfriend that I refer to as an Aunty to both my girls because she is so close to them and us. My husbands sister has a son and I don't refer to him as my nephew - more so his sisters baby. we are married her and her partner are not, I also refer to her as just Daddy's sister rather than äunty to the girls. She doesn't uphold the role, nor does she show any interest in the girls so i figure thats all she is as far as relations go. We do have a fairly estranged relationship however.
If all is cool yes you are an Aunty!

X
Sarah