We have our fair share of family drama goodness knows, but my aunts and uncles are all aunts and uncles. I don't refer to my Dad's SIL as Aunt-by-marriage
If she was lying, I think Janet's response would be justifiably more forceful along the lines "This writer is falsely claiming to be related to me and using a last name that is not her own." But since she didn't deny they are related (just not related by blood) I don't get the impression the niece is lying.