Too scared for AIBU though!
FIL fell out with his DM for a few years when DH was a child. Because of this, FIL insisted that DH didn't call her nan, but instead by her name.
So DH called her by her name for a few years. FIL then made amends with his DM and wanted DH to call her nan again. At this point DH wasn't a small child anymore but a teenager and had got used to calling her by her name.
Also she didn't act like a 'nan' in that MILs DM would see him every week, he'd sleep over at weekends, do christmas and birthdays etc whereas DH didn't really spend time with FILs DM.
Anyhoo, as a teenager sometimes DH would call her by her name, other times nan to appease his DF.
Cut to now. DH is now married and has a child. He still doesn't really see FILs DM unless at a family party for example. But MILs DM visits and we visit her.
I've only met FILs DM a handful of times. She visited when DS was born and brought with her a gift for DS. We wrote her a thank you card, to her name not nan. We don't know her address so PIL said they'd drop it off when they visit her next. Envelope says Name. PIL opened opened the thank you card to see what we'd put inside. Seen it says her name not nan inside. Refuse to drop it off til we've written nan in it. We don't have her address to give it to her ourselves.
AIBU to think that PIL should realise that DH is a fully grown adult married with a child and if he doesn't want to call his nan 'nan' then he doesn't have to?
And that FIL started all this by making DH call her 'name' when he fell out with her?