For context: PILs are mid/late 70s but a bit more ‘1950s teens’ than ‘1960s teens’. Fairly working/lower-middle class upbringing but socially mobile.
Yesterday, in conversation, I enquired after their niece. ‘Obnoxious, as usual’ came the response from MIL. ‘She refuses to call me Auntie M, and just uses my name. It’s disrespectful to her elders.’ FIL piped up, ‘children should respect their elders’. Niece is 57 
DH says they’re old-fashioned and there’s a lot more to their upset than just the presenting issue of Auntie-calling (he’s right, of course). But this just isn’t a concern in my family, IMO it’s disrespectful of PILs to take offence and pull rank. Additionally, I feel bad for calling them by their first names for 20 years, because they must think I’m disrespectful and impudent.
YABU: even 57 year olds should call their elders Auntie/Uncle.
YANBU: this is stupid, first name terms in this context is in no way disrespectful.